First off, if it was stolen, and there's no police report for it, then that's fishy. If it claimed to be purchased at a yard sale four years ago, all Carter would have to do (to prove it was stolen recently) is prove it was in his possession any time in the last four years.
Secondly, weird expensive crap winds up in rummage/yard/garage sales and flea markets all the time. Why? Because to the person selling it they don't consider it valuable and hope that someone else will find it valuable (ergo, they'll buy it).
Expensive paintings, coins, furniture, photographs, documents, etc are sold all the time.
Hell, an original copy of the constitution of the UNITED STATES was sold for $4 at a flea market.
http://www.snopes.com/luck/declare.asp
So, yes, I find it quite believable. And a favorite thought on it is: You don't know what you've got until it's gone.
Sykotyk