unidentified items

when i sell an item on poe trade, should i identify it before or not? what is the advantage of not identifying an item?

The vast majority of items should be identified (random rares with no special qualities). There are some cases where you may not want to do so.

The primary one off the top of my head is Watcher's Eyes. Unidentified ones have a certain value. Once ID'd that value could now be lower or (much) higher depending on the mods.
Another is unveiling Paradoxica. Again if the unveil is a desired one the price will skyrocket but if it's bad it would be worth less than if you had sold it unveiled.