I sell all of the basic stuff you need to play. Health potions or ammunition or whatever, depending on the genre—whatever the one thing is that you actually buy regularly. I sell it cheaper than every other vendor, too, and my stall is closest to where you first appear when you fast travel to the market.
The catch? My stall's closed for plot reasons. You need to do a fetch and rescue quest to save me from kidnappers for me to open my stall. You can't fast travel to the market after you kill my kidnappers, you need to walk all the way back to the market and keep me alive against waves of attackers along the way. Afterwards, when I open my stall, you need to go to a bunch of far-off locations and solve unintuitive puzzles, the answers to which you will almost certainly have to look up on the wiki; this will increase my favour towards you and gradually work towards the full discount for the shit you need.
I am the best and worst vendor.