One needs to remember that the number of the units on the used market is tied to the number of new units sold. Rogue has been around for 27 years and is well known for their reasonably priced amps and excellent sound. That's a recipe for selling a lot of gear. And, if a company has sold a lot of new product, that naturally increases the number of units on the used market.
And, since Rogue makes moderately priced amps, it is no surprised that people sell them as they move up the food chair to fancier, more expensive units. I've never seen much in the way of negative comments about the brand, so, for the above reasons, I wouldn't worry much about buying a used one as long as the seller is reputable.