From Diablo Wiki
Comps means "comparable." This term is used to describe items of equivalent quality and value, generally in regards to searching by price in the Auction House.
Players may wish to view the comps to valuate their newly-found item, or to consider how much it will cost to upgrade from their current gear. The Auction House has a fairly easy way to do this; just right click on an item and select "See similar items." This will automatically display comps, though players generally need to adjust the variable fields a bit to focus on the important, price-setting modifiers on an item.
- See more Reference terms.
Comp has many other potential meanings, including as an abbreviation for "computer."
Another common use of the term is to refer to something given free, or the act of giving it. For example, "They comped us our meal since my friend owns the restaurant." This meaning isn't often used in Diablo 3, so there isn't much risk of confusion.