They mean well, of course. It's generally an expression of impressed surprise. I do, as it happens, have a wide variety of merchandise. I worry sometimes, however, that they think I'm a reseller, hawking other people's artwork as if it were my own (don't be one of those people, they suck). To make the confused customers feel better, I usually try to have a multi-person anthology for sale at my table. Then I can say, "I worked on this one, but it technically has other people's work in it, too." That seems to satisfy them.
Rock on, guys, and never stop drawing.