At 6'4", you'll play hell finding a bike on the used market. Patience is going to be your ally. You'll likely have the better chance on ebay for the sheer number of bikes offered there. But the prices can jump to stupid pretty fast on ebay when you only have $500, so watch that budget.
Heres a tip: use esnipe.com and simply max your bid at $500 (including shipping) on suitable bikes. Dont sit waiting on auctions.
Also, expect that your seat to stem length will require some fine tuning once you have a bike - big torso riders often have to play with this dimension.
Try the LBS, but again, the size issue may be seen there, too. Craigslist is also another option, although it can be hit or miss, depending on where you live. Some areas have a lot of bikes, others are nearly devoid of them. Add that to your requirement for a maxi-sized frame and it could be a cycling desert on Craigslist.
All said, considering your size, you will have to keep at it when shopping used.
For $500 new, you'll likely have the best luck with BikesDirect.com, Roadbikeoutlet.com or Sportymama.com. Youll be getting a "generic," off-name bike. But dont worry, you wont know the difference between a "lesser" brand made in the same factory as one of the big names. Fact is, you'll probably get more for your money.
Last edited by dahut; 07-31-10 at 08:35 PM.