I haven't seen any change in price depending on which character is interacting with the vendor.
You get 40% of the "true" value the game displays at a shop
But you pay 250% of the "true" value the game displays at the shop as well
So you can get 300gp for a 750gp item and then try to buy back the same item for 1875gp
- No difference in prices per character (16 CHA and 10 CHA)
- No difference in prices per vendor (only vendor's I've found are at druid camp, so haven't checked this anywhere else)