Frequent border crossing and importing Mexican craft items?

There is no exemption from duty for items imported for resale, even those of wood, handmade, etc. So, anything taken into the US for resale will have duty charged.

Also, do a LOT of homework. Admittedly we have made some nice money selling exactly the items you are referring to on eBay, but in some cases, people with connections with the artisans can undercut the prices very seriously.