For anyone interested, I figured out how ego effects trading. You can find the specifics (in more detail than you're likely to need) here: http://cavesofqud.wikia.com/wiki/Trading_mechanics. The most important take-away for most players is that the lowest attainable levels of ego hurt you substantially more than high levels of ego help you, at least when trading. So 10 ego characters are not a good idea unless you plan on never shopping, but an 18 ego character isn't much worse than a 21 ego character.
If people think it'd be useful, I'll figure out how item modifiers (slender, reinforced, ect) influence prices, and maybe some other nice things like a commerce-per-pound table of early-game items.