Particularly with Mossberg, there are several models. When shopping, whether gun-show or gun-store, it will help the prospective buyer to have internet available. In this way, one can reference a manufacturer's website as well as gunbroker.com.
As I was curious about the initial question (I presume the gun-show is now over, but perhaps the original poster obtained the seller's contact info) and looked at Mossberg's website. MSRP for a similar gun could be ~$800. But if the 590 was really a 590A1, then MSRP would be closer to $900. But who buys at MSRP unless something's rare? That's where gunbroker.com would come into play.
For the original question, the answer would be "not enough info". There wasn't even a photo provided for us to begin to estimate a used price.