a good deal is only what you think it is... if you find a $500 used bike (and that's a good price point for used bikes, lots of nice ones in that range) and you like it and you feel like you got a good deal, and you enjoy riding it... guess what?? you got a good deal... a "deal" is relative to your own parameters and the market in which you live...

generally speaking, in your budget range tiagra is common and 105 components are not rare... you definitely do not want something built prior to 2000 (my opinion), avoid carbon, and look for nice clean AL bikes... good chance you'll have to look at a few, you HAVE to be prepared to walk away if you don't think you can get it at the price you are willing to pay... don't lowball on CL responses before you see the bike, that is an instant turn off to the seller...

fwiw some LBS's now are making a good biz out of the used market, taking trades, and etc... there may not be a warranty, but I'd be willing to bet they have at least gone thru the bike and given it a basic tune up... they are worth checking out if they deal that way...

in the budget you are looking at, trek and giant both have excellent offerings on the used market that will serve you well for a long time... consider some used cannondale and specialized stuff as well...