So you went to a store were employees make fun of paying customers? If that was my business they'd be fired and possibly sued if I could find a way...
That would be considered friendly banter here. ****, when I go to buy a game or movie, the clerk sometimes tells me if it's a good or bad game/movie. In his opinion, of course, which he's entitled to. I doubt you'd ever be able to make a case out of someone giving his opinion on a matter. Company would chalk it up as "guiding the customer" and they'd probably settle it internally by reprimanding the guy.
When I worked in retail, I'd often tell customers to grab an extra apple or pack of crackers or whatever and cash in the discount. Company technically lost money on it (assuming they could sell everything in stock), but customer service is valued higher than selling off as much crap as possible. Same applied when I sold fireworks one winter. I was directly told not to sell too much crap to the customers, even going so far as to suggest they don't buy certain products. People left with quality products and as such were more likely to return.
Sure, if he's telling you not to buy it and not offering an alternative or explanation, he's probably overstepping some lines, but hardly anything worth suing anyone over. After all, he did sell him the game without too much of a hassle.
Socializing 101 #3: "Haters gonna hate." Followed closely by #4: "Don't feed the trolls." For those wondering, #1 is "People are stupid" and #2 "Don't argue with idiots"