Confirmed that this achievement will not pop on my online characters when the game mode is created as RoTW. After leveling one character to 51 and the other to 79, I still did not have the achievement and I make MANY trips to town to sell almost EVERYTHING. To confirm, after hitting level 79, I then started to document each trip to the vendor and sold over 1,000 items on a single character (again, this was after making over 20 million selling to vendors before I started to keep track). Based on a Steam forum post, I created a new character on “Expansion only” game mode and he obtained the achievement at level 7. Other achievements have worked on RoTW game mode, but this one failed on multiple characters.