What is the highest tier weapon/armor that is purchasable?

The 'time passed' and your 'level' don't appear to affect what is sold by vendors. Vendors in some locations do sell much better gear (armor and weapons) then vendors in other cities.

(Spoiler alert) If you want to buy high level gear, look for vendors in higher level zones like black desert city, but some of the best gear is carried or worn by bosses, and not just the high end bosses. When I kill the Red Saber boss (in their SW Swamps HQ), he has both times dropped a Melitou saber, which is the best sword level possible in the game. He and his gang are easy for even mid level groups to take out.