Honestly, if you're going to be serious about the purchase...I'd ask to take it to a competent gunsmith and have it looked over. Presuming you mean Sears make, who hasn't made guns in a few years now, so it does have a few years behind it.
After the price though, of the shotgun and a gunsmith, though, you may wish to look around at others. Remington 870's start at around $300 new, so you might be able to find a used one in a similar or better condition. Also, if you're near a Gander Mountain, its worthwhile to check out their used guns, because those come with an inspection by a gunsmith and one year warranty.
Ok, here's the info I was able to find on that gun. I can't say if it is 100% dead on, but it's what I was able to find.
The Sears Model 21 was actually made for Sears by High Standard. The same shotgun was released as an actual High Standard and was known as the "Flight King." That particular one was manufactured between 1960 and 1966, so I would assume the Sears Model 21 was made around the same time.
In "good" condition the approximate retail value would be $125 to $150.
Again, don't quote me on any of that, but that's what popped up when I looked.