Boy, that's a tough one.
Legally, anything you take to the US for the purpose of reselling has to be declared and duty charged. That's what a customs broker will do for you.
Small quantities??? Depends. We have taken back items to sell on eBay. We make a point of having them well wrapped, and put away in the basement, closet, or just out of sight, but not "hidden." We have been careful not to have TOO much-stopped at 8 catrinas, we could pass them off as family gifts. Stopped at 80 yards of 8 kinds of Day of the Dead fabric, for sewing and quilting projects. That kind of thing. No cases of identical items.
Our license plate has a very long history of crossing the border, 36 years worth. It shows that we have never had an issue or attempted to enter with anything we shouldn't. That certainly helps. We also try not to use crossings where you are guaranteed to have to go to secondary in an RV like Tijuana and Nogales. It's just less wear and tear on ourselves, no matter what we are bringing back.
We haven't been questioned about any of our purchases, ever.
