Do they buy the goods with Venezuelan currency?
I doubt that Venezuelan currency is accepted in Colombia. I’d marvel at the prospect of anyone even offering an exchange rate, given the wild inflation. These Venezuelans crossing into Colombia to buy staples must have other currency, or they’re selling something to get it once there.
I am sure no store in Colombia would accept Venezuelan currency. They must have dollars stashed someplace. Or some items they can sell in Colombia.
Probably US dollars.................