For each 10 trade skills you can buy items one level over your character level. Above item level 100, you need 20 trade skill for each extra item level. There are a number of ways to increase the cap on this skill:
- The Black Market Relic - increases cap to 180
- Huckster Friend - increases cap by 20
- Golden/Silver/Bronze Neckalce of Trade can boost past the maximum trade skill by 30/20/10
If you gain the black market relic before huckster friend, you can have a maximum level of 200 (boosted by 30 to give 230), however, if you get huckster friend before the black market relic, you're stuck at 190 trade (boosted by 30 to give 220 max)
Despite the description, it seems the black market relic simply increases the maximum of trade by +10, so it doesn't matter if the AAC is bought prior to the relic. (Confirmed by only 1 player)
Learning level: 12
Max level 170