A photo from the Muppet Wiki website showing Jim Henson with the Ideal toy puppets of Rowlf and Kermit that were marketed in 1966. (An interesting TV trivia note: 1966 was also the year that the original Star Trek and Batman TV shows made their debut!)
Sam and Friends
When "Kermit" first appeared on TV in 1955* on Sam and Friends he was simply a lizard like puppet, and because TV was then seen only in black and white, nobody knew he was green (unless this fact was mentioned during a broadcast or in a news article). Aside from possible news clippings about the program, there does not appear to have been any collectable merchandise produced for this show. However, this was where it all began!
*Update: The Wikipedia website states that Sam and Friends made it's debut May 9, 1955 and ran until Dec. 15, 1961. https://en.wikipedia.org/wiki/Sam_and_Friends
Kermit and Rowlf (the only "Kermit" plush toy)
Kermit's long time Muppet pal Rowlf the Dog made his TV debut in 1962 for a commercial about Purina dog food and in 1963 made regular appearances on the Jimmy Dean Show. Shortly after, Kermit and Rowlf were reproduced as toy puppets by Ideal in 1966 (shown above). These two toy puppets have now become "Holy Grail" items for Muppet collectors. As Kermit was not yet a frog, this plush toy is the only Muppet collectable to be made in which the character is simply "Kermit" without his collar and flippers. On rare occasions when the Ideal Kermit puppet turns up in auctions on e-bay it has been valued in the area of $1000 or more if it is in mint condition with the original box. I've seen it sell for hundreds of dollars without the box. The Rowlf puppet also sells for several hundreds.
The Muppet Wiki page has additional images of the puppets and their original boxes: http://muppet.wikia.com/wiki/Muppet_puppets_(Ideal_Toys)
The puppets are also shown on the Muppet Central website: http://www.muppetcentral.com/collectibles/muppets/dolls.shtml
Other Muppet toys from the era
Some other characters that Jim Henson used in TV commercials during this period also provided some collectables.
The two Muppets characters that were used in the Wilkins coffee commercials during the late 1950's were marketed as plastic moulded puppets and as flat printed fabric puppets. They're names are Wilkins and Wontkins. These are the very first Muppet toys! The Wilkins coffee containers also had an illustration of the Muppets on the lid. http://muppet.wikia.com/wiki/Wontkins
In the late 1960's a three foot tall inflatable figure of the La Choy Dragon was marketed though it looks little like the Muppet character. http://muppet.wikia.com/wiki/Delbert_the_La_Choy_Dragon
In the mid 1970's Henson Associates designed a commercial toy puppet of a kangaroo that was marketed through the Continental Bank. A "real" Muppet of the kangaroo was never made, only the toy version which is quite unique for the Henson company. The toy puppet was featured in a brochure that was distributed to customers at the banks and there was a full page newspaper advertisement featuring an illustration of the puppet. I'm not aware of what the value of this puppet is, but it is quite rare.
Becoming a frog
It wasn't until the mid 1960s, after colour TV came about, that Kermit became "Kermit the Frog" for the TV special Hey Cinderella. Originally intended to be a children's series, the pilot was taped in 1965, then the project was revised to become a stand alone TV special which was recorded in 1968, and it was finally broadcast in 1969 and 1970. This was followed by The Frog Prince (1971) and Muppet Musicians of Bremen (1972) both of which also featured Kermit the Frog. All three of these productions were recorded in Toronto, and were the first of a long list of Henson productions to be recorded there.
In 1970 when Hey Cinderella finally aired, a series of cheap plastic bag style hand puppets were produced and sold as a special offer through Hawaiian Punch drink mix. These puppets were simply an illustration of the character printed on a plastic bag shaped like a hand puppet. They were similar to those made popular by McDonalds restaurants during the same period. (McDonalds offered hand puppets featuring the McDonaldland characters as a free give away to attract families with their kids.) To my knowledge Kermit was not included in the Hey Cinderella set of puppets. http://muppet.wikia.com/wiki/Hey_Cinderella_hand_puppets
Here is the Video Disc release of Emmet Otter's Jug-Band Christmas (originally produced in 1977) and The Muppet Musicians of Bremen (1972) that was marketed in 1981. They were packaged together on the same disc which is titled Tales from Muppetland. Below is the back view.
Kermit and the debut of Sesame Street
After appearing as a Frog for the first time in Hey Cinderella, Kermit then appeared on the very first episode of Sesame Street in Nov 1969 as a central character. Although he was a regular character on the program, Kermit was not included with the cast of Sesame Street Muppets when they were marketed in the early 1970s as toy puppets, dolls, puzzles and more. However, Kermit did appear in a few early Sesame Street children's books, and the character's soon to be famous song Being Green was included on several of the Sesame Street record albums. Yes, it seems strange today however the song was first recorded for Sesame Street, not The Muppet Show! This same series of records also included albums based on the TV specials of Muppet Musicians of Bremen and Frog Prince (shown below) which featured Kermit the Frog.
This is one of the first Sesame Street books to be published, and it includes Kermit the Frog. It was published in January 1971 by Signet, roughly a year after the debut of Sesame Street. It is from a small series of similar sized Sesame Street books offered by the publisher.
Here is how Kermit appeared inside the book, next to the letter "F" for frog!
A 1972 Spiegel advertisement showing the first wave of products from Educational Toys/Topper. Shown here are three Sesame Street hand puppets, five finger puppets, a Big Bird doll, and "Walking Letters" sets. Image found on the Pinterest website. Note that there are no Kermit's among them! (FYI, Spiegel is a direct marketing company in the US which sells products through catalogues. Here is the Wikipedia page about the company: https://en.wikipedia.org/wiki/Spiegel_(catalog) )
In terms of merchandising, Sesame Street was the first run away success for Jim Henson. He had experienced popular success with Sam and Friends (1955), and then broke ground on licencing his characters as a result of the commercials for Wilkins coffee (c.1957) and Rowlf's run on the Jimmy Dean Show (1963). Yet Sesame Street (1969) was the first Muppet program to be so extensively mass marketed. Henson had retained ownership of his Sesame Muppet characters, which allowed his company to share in the profits of Sesame Street licensing with CTW, the producers of Sesame Street. The volume of Sesame Street toys and other household items to be produced in the early 1970's and onward no doubt prepared Jim Henson for marketing The Muppet Show in 1976.
A 1977 advertisement from Kickerbocker for their line of Sesame Street plush toys that was introduced in 1975.