What would worry me would be that if its a lemon you won't be able to tell and the experience of playing such an instrument could put you off learning it.

If you want to learn the banjo you'd do better to save up for an entry level model (they aren't that expensive) or hold on until you find a cheap one you know is in decent nick.

Having said that if the one you've spotted has "White Laydie" written anywhere on it you might want to punt thirty quid at it..!