Why was Tuco Salamanca willing to buy crystal meth from Walter 'Heisenberg' White & Jesse Pinkman?
I mean, obviously he was a distributor of crystal meth and they were manufacturers of some fine quality product. But surely his first thoughts must have been on his Uncle Hector and their mutual connections with the Juárez Cartel, who were presumably already smuggling crystal meth across the border into the US. Heck, his uncle killed Maximo Arciniega for the affront of Gustavo and Max 'distributing'1 on their turf, combined with maneuvering them into a meeting.
I'd have thought Tuco would have liked to take them out on first hearing they were manufacturing, or if he realized the quality of their product before doing so, smuggled them across the border into Mexico to teach the cartel cooks (then kill them).
1 I fully accept Max & Gus's claim that they only gave meth to cartel personnel in order to get a meet with the bosses. Having a cartel to distribute their product would have made their business venture slightly less profitable, but a whole lot easier & safer (with cartel henchman both distributing and enforcing against competitors and defaulters). Further the cartel could probably move a lot more meth in a day than Los Pollos Hermanos could in a month. A moderately lower profit off a much greater volume means bigger money in the end.