What farming means in this game
Gold by the million, the three kinds of material, the mythic uniques that only drop off ubers, and the paragon levels sitting behind all of it. This page collects the things that are a time problem rather than a skill problem, which in Diablo is most of what people buy.
What to buy first, honestly
If the character is not sixty, levels. If it is sixty and the gear is thin, the Pit for masterworking materials. If the gear is finished and the damage is not there, paragon. Buying in that order costs less than buying the expensive thing first and then needing the cheap one anyway.