Suspicious of my girlfriend's AR Sale

Okay so long story short. My current girlfriend's husband died 4 years ago and she inherited his arsenal. Her neighbor has been really bugging her to buy some of the guns, particularly her AR. The AR in question is self-built with a Palmetto SA receiver + 12 "clips" (I assumed she means magazines). He offered 600. She countered with 800 and he readily agreed. Thing is, looking up value I see these sell all day for around 400. She doesn't know guns, and she doesn't know the guy at all. Is 400 truly the value of these guns, and should I be suspicious of someone willing to pay 800 for this gun?