Pretty simple - supply and demand.

Supply: There tends to be overstock of easily gained EIs which takes a long time to solve itself, if ever, depending on the amounts by which they were gained. Easter '10 Eggs are a great example of these. On the opposite site, you have one-item-per-account items, because that means at most one spare (one mule with item) for people to sell.
CIs in months with bonus-item tends to be slightly overstocked as well, especially if the bonus item comes only per x sets/CIs bought with x in case of the number of sets higher than 1 (or sole CIs higher than 2), and the month directly after tends to be in far lower supply because most people spent extra money the bonus-item month and thus don't have the money to buy as many CI sets as they usually do in the month after.

Furthermore, the older an item, the rarer it tends to be, because a lot of them are "locked up" in the meaning of people having only one, which they don't want to sell. This especially goes for items around the time Mene still had only a small number of people - such as the earliest CIs and EIs.

Demand... based partially on the quality of the items (well-done items sell better than poorly done) and their theme (items that combine well with other items or with common themes for avatars sell better than items that only are interesting to a small group of people/are limited in the ways they can be used), partially on their rarity (some items are sought after as much because they are a status symbol as because of the item's actual merit), somewhat influenced also by the time of the year or events. Around winter, wintery-themed items tend to be sought after for fitting avatars. Around Valentine, love-themed or pink/red/purple-coloured items are sought after because they fit well with the event.

Furthermore, gold also plays a role. The more gold is available in the community (and the easier it is to get) , the higher the prices can be. You generally don't put something up for sale at a price no-one can pay. This is why gold sinks, such as the commons, are important. If gold keeps stacking up, inflation becomes worse and worse.