I don't know if this is a bug, user error, or a suggestion - perhaps all at once!
But as someone playing a PC with a decent Appraise, one of my biggest peeves is when I see someone else coming up to a NPC merchant when I'm in the middle of selling off a haul - because as soon as that other PC opens up the NPC's dialogue and store, whatever amount I was getting will reset to whatever that person is getting.
Example: last night, I was selling off things at the Peddler in Cordor. My max was 670, which is my average with 83 Appraise.
Someone else comes up to the Peddler to do whatever, and the next thing I sell was sold at 311. I had to wait for that person to finish, then redo some of my skill boosters to get back to 670, and then I could finish selling. Thankfully, it was rather late at night, so no one else approached. I can only imagine the frustration in trying to sell at a popular vendor in prime playing hours.
Should I be doing something different here? Is this a bug? Is there a way to stop this from happening each time someone new opens the NPC vendor? Or is this one of those "that's just how Arelith is" things?