First of all, it's nearly impossible to render any kind of verdict without more and better photos. Most experts (I am not one) are very cautious and will be reluctant to voice any opinion unless and until they can be sure of what they are talking about. And it's tough to be sure of much with only the picture you've provided.
However - there are some basic things you can do, and information you can furnish, to get off to a start of sorts:
1) BIGGIE - do all numbers match? I gather from your post that you have not disassembled the gun to find out. If you have access to it, and if the owner is amenable, find out how to disassemble the gun (not hard - there is lots of information on lugers around), then carefully take it apart and make sure all numbers match.
2) What, if anything, is marked on top of the barrel's chamber?
3) What, if any, markings are on the top of the toggle?
One thing that is immediately obvious is that the condition of the sideplate appears far different from the rest of the pistol (and I don't see a number on it - is there one?). This, even to a non-luger guy like myself, screams that it is not original to the rest of the gun (or was not treated the same as the rest of the gun if the gun was re-blued), and if so, that alone makes value drop precipitously.
Your best bet is to 1) get more and better photos and then 2) take the photos and show them to the experts at Jan C. Still's luger board, in the "New Collectors" forum. The guys over there are very experienced and knowledgeable. You will not go wrong listening to them. Here's the link to the forum:
http://luger.gunboards.com/index.php