One of the biggest misunderstandings in putting a value on Barbie is the stamp “Mattel Inc 1966”. This stamp is the same thing as any website that announces their copyright to the content, starting from when it was first published. The stamp on Barbie is the year the TNT body type (twist n’ turn) was first used in Barbie, and was widely used from the late 60’s to early 90’s. Therefore, if you have a Barbie with the year 1966 stamped in the back, you’ll need to find out more about the doll to determine whether she’s valuable or not. (The same goes with Ken.)
You will get instantly closer answer to the age of your Barbie by checking if she has a stamp in the back of her head, just in the seam joining the body to head. There has been several big revamps of Barbie’s heads; One in 1976, in 1985, then again in 1998. In a rare case the 1966 stamped TNT or Shani body is joined with a larger head type that collectors disrespectfully call BHS which means “the big head syndrome”. The large headed dolls are never older than from 2006 regardless of the stamps.
Boldly generalizing, Barbies stamped with 1966 are worth approximately 20 US dollars mint in box, so a played with doll is not worth much more than 4-10 dollars depending on the condition of it. There are some individuals that were so popular when they were new, that they have more interest value than normal. (Rockers-series, Peaches & Cream, Dream Glow etc.) Also, ironically, the cheapest dolls (such as Fashion Play -series) that were not really valued at the time and not often saved are now the ones that go for high prices because they are rare finds in good condition.
“Patent Pending” or “US Pat pend” stamps are all markings of an actual vintage doll.