I'm judging you.

I speak to the sellers politely, yeah, using -desu/masu forms, but I don't speak as politely to them as they do to me, because I'm the customer, technically. I'll use moraimasu when they'll use itadakimasu, I'll use just a -masu form when they'll use o-____shimasu. Haven't had any complaints yet XD

But when in doubt, be overly polite, I guess :D