The standard advice if you're buying as a novice is buy a new instrument, then if there's something wrong with it you can take it back to the shop. If you know someone who knows about instruments, take them with you to help in the decision.

My best advice, after years of playing, occasionally working in music shops doing set-ups and repairs, is to find a good repair man who sells second-hand instruments and discuss the matter. He's the person who will spot a bargain that's not correctly set up or needs a small fix. If he knows you are ready to buy, he can take an instrument that he wouldn't normally risk being stuck on his wall for months, knowing that his profit is assured. Just make sure that you get the right man, and be prepared to wait until he's found something. And if you find a banjo you like, tell him about it so he doesn't waste his time, or slip him $20 and ask him to check it out when he's passing the store.