Merchant for a day is a lvl 65 sidequest in Azim Steppe. I really enjoyed it, as it reminded me of Dragon Quest 4 where you also played merchant for a bit. I failed it twice at first but nailed it the third time.

There are some really fun sidequests in this game. Just kinda sucks that you have to slog through the many fetch and/or boring ones first to get to them.