Yesterday, fellow Trade Academy pilot Zach Starcrusher and I had a discussion on trading goods. When I just started out in Eve, I used to sell everything if the highest buying offer was above regional average - in other words, if behind the price the green text " xx % above regional average" was printed, I would sell immediately.
After a while I realised, that I was selling to the highest offered *buying* price, which is often much less as the price other eve pilots are willing to pay. I was selling stuff way under the market price, effectively spoiling the market for other pilots at my own expense. Doubly so because there are pilots who will offer ridiculously low buying prices for certain items, in the hopes that n00b pilots will sell. Yesterday for instance I had a buying offer of 150 ISK a piece for Caldari Navy rockets, which I sold for 1800 ISK a piece an hour later. The price difference can be huge!
After that, I started pricing my goods more or less in line with what others are selling for in my station, system or region. Remember, people are often lazy or in a hurry, so distance between available objects plays a role in pricing: supply and demand on the levels of station - system - region. If an item is for sale in my station, I'll sell for about the same price others are asking; a bit lower if I need cash, a bit higher if I have time. If an item is for sale in my system but not my station, I'll sell for a slightly higher price, and if an item is one or more jumps out, the price goes up a bit for every jump.
This system has brought me much more ISK than I would have had if I were to sell for the highest buying price. To be honest, I've never been much of an economic or merchant, so if other pilots have better systems for selling, please share so that we all make more profit :)