There are addons which will let you see average/recommended prices for selling things to other players in the items tooltip. I can't remember for sure but I think you can see that before you pick an item up.

Tamriel Trade Center is mainly for running an online database of guild traders which makes it easier to find things you want to buy, but it also adds recommended prices based on current listings to item's tooltips.

Master Merchant and Arkadius' Trade Tools are intended to give traders more info on their own guild stores and sales, but also add prices to tool-tips. Unlike TTC they're based on sales in your guilds stores. The advantage of that is the price is less effected by speculation because a sale only counts if someone buys it (whereas TTC is based on listings so can be affected by over-priced items which will never sell|). The downside is it's a much smaller pool of data, especially if you're not in a dedicated trading guild, so fewer items will have data and what's there may be out of date or based on a one-off sale.

I use both TTC and ATT for different reasons, but because I'm in a social guild which usually has a trader rather than a dedicated trading guild I usually find TTC more helpful for prices.