Buying:
Best
Multiple Henderson cards.
Worst
Can't say that I have. I've had a few cards come back that were not "as described", but that's about it.
Selling:
Best
An old Epson 777i Inkjet printer for $85. I bought it on eBay a few months earlier for $50.
Worst
Sold a digital camcorder that was working perfectly fine when I shipped it; however, something must have happened in transit as the buyer claims it didn't work once he received it. Had to go through the whole "insurance" mess and stuff, and it all worked out, but it was a major pain in the ass