Department stores have always sold junk bikes. You just need to know the names of the less desirable brands (there are exceptions though). Generally any road bike with a one-piece crank isn't worth much, (although some people do like the old Schwinns, despite the 1 piece crank).

The other issue, if you're thinking about buying bikes to resell, is you may want to add new bar wrap , cables, housing, tires, tubes, etc. to get top dollar, and all of that stuff adds up pretty quickly if you don't have access to it wholesale. Flipping bikes profitably is harder than it looks, especially since you'd be competing with people that do it just because they love working on bikes.

Some of the brands to steer clear of (YMMV, obviously) are:

Huffy (except the Serotta Huffys, which are very desirable)
Murray
Magna
Roadmaster
Free Spirit
Pacific
Motiv
Next
GMC Denali

etc.