I would be hesitant, Parker made a great observation on the grip and the possibility of the broken trigger guard ears.
He says its a Spikes Tactical, but as of right now Spikes Doesnt offer any barrels longer than 16" other than the Lothar Walther 18" Fluted 'SPR" barrel, That gun has a 15" Alpha rail on it, so the Barrel is 18" but not fluted so I cant say if its a Spikes Barrel or not, it could be a discontinued model factory built from Spikes, or it could just be a striped upper receiver he bought from the spikes website I dont know.
If its a Factory produced gun, or something he bought new at a gun shop he should have the receipt and build sheet telling you how much it cost and what it actually is.
The scope is crap, I think its an Older Barska "sniper/tactical" POS, Im not sure, I am sure that its not anything good or worth keeping on the gun.
There are a million Fake Troy BUIS on Ebay, so I would want really good pics and a receipt for those as well.
The stock is Magpul with the enhanced buttpad, But again could be a PRS product (the airsoft version) ask for good pics, they are stamped PRS so there is no confusion, they look identical but are made with an inferior plastic, not the great Dupont FRN.
The Troy Mags are decent, I dont like them because I broke one dropping it onto pavement while loaded from about waist height, But should be fine for flat range practice or target shooting. But they are like $10 a piece so shouldnt be added into the value of the package.
Best Case Scenario you are still looking at a Used AR with a crap scope, and I certainly dont think its worth $1100
Breaking it down into parts
15" Alpha Rail $220
Troy BUIS $180-220 (if they are real)
Magpul ACS $90 enhanced buttpad $20 (again if its not PRS airsoft stuff)
here is where it gets tricky
Lower $150
Lower parts $100-400 depending on trigger
Upper $100
Barrel $150-600 depending on what it actually is. Its at least 18" and it looks pretty thin profile, so I am going to assume the barrel is fine but on the lower end of the price spectrum.
Bolt/Carrier $100-220 depending on who made it and if its an AR-15 carrier or an M16 carrier.
So I would get some straight answers before you make any trade, that is a terrible ad with absolutely no information on the gun other than its an AR-15.
But making a gut call on It I would say dont make the trade for what you offered.
The Aimpoint Comp m4 you have is certainly worth the $500 depending on its condition maybe more (since you got it for $500, im assuming its not mint so $500 is fair)
Your Chinese Mosin Carbine, again depending on the condition is a what $100-120 rifle?
So right there without the AK mags or pouches you are at entry level AR prices.
Honestly I would try to keep that Aimpoint for the AR once you get it. I have made the mistake of selling/trading good optics before and really regretted it later when I went back to buy a cheaper version of the same damn thing I already had.
Even if that Comp m4 is beat to hell and works its honestly worth hanging on to, because it will keep on working.
If you keep your eyes open for sales and are patient (plus are reasonably skilled with your hands and have the tools) you can build yourself a very nice AR for about $550-600, I built a lightweight AR with a spikes 14.5" barrel for about that
I got all my stuff on sale over a period of a few months, and built a mean little rifle that I would stack against any factory gun.
I hope this helps