Always a tip off. When a "store" is offering high retail or even half that for a used item, they probably aren't a legitimate business.
Legitimate businesses have to make a profit, and there has to be some meat left on the bone.
A local sting many moons ago made the same screwup by offering one fellow retail for a used Atari Game. While the game was broken, it was the property of the seller, who promptly took the offer. The word hit the street that something was fishy within the hour.
According to the rest of the story, the criminals knew it was too good to be true, but the momey was so good, they could not resist.