Basic advice
If you can, take a more knowledgeable friend with you.
Make sure it fits! Lots of people are willing to settle for a used bike that almost fits. Would you settle for shoes or pants that almost fit, even if they were an incredible deal? No—don't buy a bike that almost fits either. Get one that does fit, you'll enjoy it much more.
Otherwise, make sure you try to move that seatpost. Make sure the stem moves if it's an older quill style. Look for signs it has been left outside—rust on the axle ends, hints of rust on the steerer, rust on the chain etc. Look for mismatched parts, like if everything is 10-speed 105 but the rear deraileur is an older mtb unit something is funny. At that price range I'd stay away from anything beyond a pristine looking carbon fork with aluminum steerer and axle end, other carbon stuff is either going to be new and cheap or old and abused, neither of which is a good idea.
New bikes in that price range do exist, and might be a more solid choice especially if you aren't terribly bike savvy, as while it's easy to get a good deal on a used bike there are some unscrupulous sellers out there.
One new bike you may want to look at is the Torker Interurban. Thick carries them.
http://urbanvelo.org/torker-interurban-first-impressions/