Post a thread here titled"THIEF WARNING do NOT buy eggs from _____" then explain the situation in the body... other BYCers will appreciate the heads-up, and this thief will lose credibility.
I would also email them that if the money isn't returned within x-number of days, they will next be hearing from your attorney and then law enforcement - tell them an atty can file subpoenas, and the courts will issue summons, etc etc... at that point, you will be holding them responsible for costs of investigation, damages, court costs, and whatever you can think of to rightfully tack on to that amount. They are hiding behind this e-mail/paypal anonymity as their protection... understanding that you are willing to do what is necessary to take that away on principle will make them realize you are serious, and that NOT responding will cost them a heck of a lot more than the $63 they owe you...
The bottom line is that this is stealing.