As a lawyer/ litigator licensed to practice in 3 jurisdictions I am paid to think about these things.
I feel comfortable that in a lawsuit where the seller advertised an UNRESTORED book, that was revealed to be RESTORED, and he AGREED in writing to send back $300 following return of the book but does not- that is an enforceable verbal contract- an exchange of promises and he would be liable for, at a minimum, the extra $100 plus court costs. FYI, these were promises made in a back and forth ebay message conversation - not a message board. They would have been specific statements directed to me that I was intended to rely on.
Now whether or not I could collect on a judgment, whether its wise to start a lawsuit over $100, etc. are fair points and issues that may make a lawsuit ultimately not worth it.