RE: price on a 20ga Wingmaster
Button,
870's can be purchased new for under $300.00. Now those would be the express guns and not the wingmasters, but the only difference between the two is the wood, bluing finish and scroll work. If all your son is going to do is hunt with it then it makes absolutely no difference between the two.
That being said $200.00 otd is an ok price depending on how rusted the receiver is and how good mechanically it is. Check it over real good (hard to break them anyway) but make sure it functions as it should.
If you're not in a hurry then you can probably find one in the $150.00 range, but then again if it's the one you want and it's there I'd say go for it, it's not a bad deal especially if you can get the slug barrel at a good price.
Good Luck