There's no Appraise skill in 5e, and appraising in terms of assessing an item's value would likely fall under different skills/proficiencies for different items (e.g., Arcana for scrolls, or maybe History/Smith's Tools for weapons/armor). As far as negotiating prices is concerned, Persuasion would probably be the best fit overall, but cases could be made for Deception or Intimidation, so maybe using the best of the three would be an option.
My main concern with skill-gating item prices is the inventory shuffle between characters that it can require depending on how it's implemented and the potential frustration of any associated dice rolls (unless they do something like scale savings/earnings based on proficiency bonuses). If they implement reputation for areas/factions (or if they have already without me noticing), basing buying and selling prices on that could be a reasonable approach, as could basing the benefit on having at least one party member present who is proficient in the associated skill(s) even if they aren't the one speaking to the merchant.
