selling most anything is like selling your car. If you have the time to waste selling the item, you should do so in the classified section of any major forum, this forum in particular is a great place to start.

If you don't want to deal with the headache, give it to a pawn shop or dealer, but keep in mind, they're never going to give you what it's worth - they are in the business of making money, like everyone else.

It's purely a decision based on how much time/energy you have for the sale, and whether or not your time/energy is worth the money you'll save by selling it privately.

I don't know much about these guns in particular, but I'd expect at least a difference of 50-150 between the private party sale and what a shop may offer you, for what it's worth (purely speculation).

I think they stopped making those in 1996. A quick search on my part shows prices @ $1000 and up. Depending on condition and accessories.
You could try selling it on this forum or on the P38 forum. We don't have any pawn shops around here. You could sell it to a dealer or put it on consignment with a dealer. Either way the dealer is going to make some $. Your post sounds like you want to get rid of it in a hurry with the least amount of hassel. If that is the case a dealer might be your best bet. If you're trying to get the best price advertise or put it up for auction.

If you do decide to list it on a forum, the walther forum would be a good place to consider. They also have an entire sub-section on the P88. It's a fine handgun. Don't sell it cheap!