just check each of the knobs ... when you have the chance to play on it: that's probably the best check.
also bought mine 2nd hand from private ... was all fine.
but: before i tried to bid on eBay and had the *second highest bid* ...
then the "seller" contact me via eMail and i really think they wanted to rip off my money ... (very bad translation, price not realistic, ...)
later on i read about this way of betrayal and was glad the Virus was already sitting on my desk for 15min, just before i received that suspicious eMail
also: to make a sales contract is always recommend, if it's a little more money.