I've tended to put in specific encounters and relationships in which PCs can buy or sell things at other than standard prices.
For example, in the last campaign I ran, I had an acquaintance of the PCs introduce them to a friend of his, an arcane archer who'd lost his magic bow. To raise funds for a new bow, he was selling off a lot of other magical items. They got some magic arrows, a quiver, and I think a ring of protection at half price. In another example, the PCs had a long term working relationship with the church of Rao, which made them a standing offer to buy certain low-level items for more than they could get selling to merchants (I think I gave them 65%). The church would buy standard, useful +1 armor, weapons, rings of protection, and cloaks of resistance for their elite soldiers. (By useful I mean they'd buy things like longswords or chain shirts, but would skip on things like dwarven urgroshes and padded armor.)