Upload & Sell: Off
There are plenty of debates out there covering this...
Since you said your budget is $500, I'm guessing you are going with an ink jet printer. I use an Epson Artisan 730 ( $199 but you can find them onsale for $99 sometimes) with a bulk ink system. You can get 2 of them for your $500 so you have a backup. The bulk ink system I use came from Ebay and cost me about $25 and came with all ink tanks full. So far I have about 2500 prints thru this thing and still going strong with only half of the original ink used from the tanks. The Epson Artisan 730 boasts a 10 second 4x6 print time.
These printers can house two different sizes of paper if you choose to offer choices.
I'm sure others will chime in, but this is just what I use.... you could always up the budget and go with a dye-sub.....