The Merchant Skill can seem to be one of the hardest skills to level up in. But you don't necessarily have to know how to create or paint items in order to gain skill in this category. Any item that you sell, be it an item from There or from a different designer, counts toward the Merchant Skill. If you do Design/Paint an item and it sells, the entire sale counts toward the skill, not only the profit. This counts for all Auctions. Here are the levels of T that each Level of the skill come from:

Casual: approx. $T5,000, or approx. US$2.80
Avid: approx. $T25,000, or approx. US$13.99
Expert: approx. $T250,000, or approx. US$139.90
Renowned: approx. $T2,500,000, or approx. US$1,399.00
Legendary: approx. $T25,000,000, or approx. US$13,990.00

While the amount of T$ to get to Legendary seems very high, Expert level is very reachable. My personal opinion is to auction anything that you own which could turn a profit. Meaning, if you're willing to part with it, you could get another copy of it from the income you had of the auction of the item in the first place, then List that item in auctions. Listing an item that you didnt design costs only 30T if you would like the buyer to be able to buy that item instantly off of a price you set. This is a very small fee, but once someone buys an item you auctioned and did not design, a 5% tax is taken from the auction. These are the ways that There keeps new items coming into world, these fees keep There running.