I'd also attack you on the grounds of stupidity.
Since it takes 2 workers to extract 1 sulfur and each worker could be a citizen providing 4 gold each in the same amount of time, the minimum anyone would really sell a luxury good would be at around 10-15g depending on the type. Of course marble sells the best and sulfur the worse (the only time you need sulfur is to train troops and if you're smart you're not losing enough to make sulfur a necessity). Wine and Crystal hover around the mid point with wine slightly lower due to excessive supply (1 colony supplies more wine than is needed by 4 others cities with level 10 taverns).
For every 2 levels (1, 3, 5, 7, 9, etc) you extend your viewing range by 1 island. That means at level 1-2 you have a 3x3 box around you, 3-4 you have a 5x5, 5-6 you have a 7x7, 7-8 you have a 9x9, and 9-10 you have a 11x11 box around you. I actually have all level 9 trading posts because thats just enough to reach out and touch some of the TGA hub peeps from my frontier position.
Hell, I just sold Nero a crap load of wine at 30g a pop.
Your ability to use the trading post, however, is limited by the upgrade level of your trading partners. If they don't have a large enough trading post to see your goods on sell or for sale then they won't be able to make deals with you.
BTW, the trading post is the trading post. Officially the "Trader" is the premium ability to trade resources for others at a 1:1 rate and wood to luxury goods at 2:1 at the cost of 3 ambrosia.