A short list of setting assumptions that make D&D work:
1) Adventuring is very, very deadly. Most adventurers fall down holes and die.
2) Mages are rare and secretive. They do not integrate into legitimate power structures; rather they hoard their knowledge from rivals and from a fearful world. The world that had mages mass-conjure and mass enchant now lies in ruins. people loot its' remains, usually dying in the process
3) Peasant is not the default origin. It is younger noble with no inheritance.
4)You are not buying and selling at village smithies. You flog to travelling merchants, nobles, wealthy collectors, wizards and barons.