Which .50 caliber round?? .50 AE?? .50 BMG?? .500 S&W?? .50 Beowulf?? I am sure that there are buyers for each of those calibers. Just look at current retail pricing, then come down some. So many people think that they are a retail store and are charging ridiculous prices for the firearms and ammunition. Look at several places online, then price yours accordingly (it will sell much faster if you are fair with your pricing.
If you have .50 AE and want a reasonable price for it, I would probably be interested (however, you are WAY south of Mesa, so that would preclude me from purchasing it simply because of the drive for me).