What you are looking at is the M&P 15 Standard model. Standard just means it's not one of the "kitted" rifle so it comes very M4-like - that is no fancy furniture or rail setups, no added cost sight packages, etc. It is very solid rifle, and has chromed barrel, gas key and carrier so if you want an M4 it's a great choice.
Now, you can purchase that rifle NEW for between 900-1K pretty much anywhere ($950 at Buds Gun Shop), and if you qualify for Mil/LEO/1R discounts you can get for $870 at BudsPolicySupply. Bear that in mind when assessing the used value. Did the kid shoot it a little and store it in his closet, or did he shoot the **** out of it and leave it in the truck all the time...
I agree with the others, the scope is going to be the wildcard. If some cheap low end Wally World Bushnell it really would add little value to the deal, but if it's a nicer model, then you can adjust. The other stuff your looking at $100-$150 when new probably, depending on the light - so call it $75 used. You didn't mention ammo, usually these "take it all" sales include some ammo as well.
I'll fall into the same mix as the others, $800 seems about the most I would pay, if that. I've been in the same situation with a remarkably similar circumstance, and same as you wanted to be fair for both - Sometimes people just need a reality check on what their stuff is worth. They've bought it during the "scare" so overpaid to begin with, added a couple hundo in accessories, and only put a couple hundred rounds through it so they think they can turn around and sell it for close to what they paid... That's just not the case anymore - as the other poster mentioned, it's a buyers market right now. At some point you just need to step back and say look, I can buy it new for this amount, so let's get realistic on a price. So yes I'd say $800 seems about right, maybe $900 at the most if a nice scope, or if the rifle is just basically pristine. And adjust if there is ammo included...
And just to play devils advocate - Even understanding the background of the deal - if you think you might want to change some stuff up, ie changing to a rail or modular handguard setup, or go with a Red Dot and some nice folding buis, change up the furniture, etc - then you don't want to buy this rifle. You instead should be looking for a 15T or MOE, ORC, etc that has most of that already in (or not in) the build of the gun. Otherwise you will just wind up adding $500 or more to the price of what you buy the Standard for... For instance, the ORC is a great "builder" rifle...if you look around you can find it for not too much more than the Sport, and it's ready to be dressed up however you like..
Wow, sorry that got long fast... didn't mean to ramble on.
