is a remington 870 20g. with combo barrel and a scope, a stoger 28g. double barrel, a marlin 22 mag witha scope, and a marlin 22.cal with a scope worth 900$???

By the nature of the combination, I'm guessing you're buying these guns used.

I'm not really certain what he's asking for each gun individually, but they'd be about this much new around here:

Marlin .22mag: $250
Marlin .22lr: $200-250 in a bolt, $110-150 in an auto
Stoeger 28ga $350
870 20ga combo: $550

That doesn't give any values to the scopes, but you didn't comment on their quality.

A Marlin semiauto .22lr used is basically worth nothing, maybe get $50-$75 for one if it's in new condition to the right person, a bolt .22lr and or .22mag might bring $100-125 each unless they're specialty models, then they'd be more. The 870 combo could likely bring $300 or so maybe more to the right person. The stoeger is likely worth about $250 or thereabouts. That totals up right around $900 if the two marlins are bolt guns.

By the looks of it, this guy wants to get top dollar for the shotguns, so he throws on two guns that wouldn't sell for much to sweeten the deal.

I'd say, if it seems like a good deal to you, buy it, because it's pretty close to a decent deal on EACH gun totalled up.