My name is ***** ***** I will do my best to help. In the condition I think you are describing (like new but fired) your P88 should have no problem bringing a solid $1,500. If you wanted a fast sale, I would not go below $1,300. Virtually every firearms price guide will support this price. I hope this helps. If you have any other questions, please let me know. Please remember to rate this answer using the stars.
Have a great day,