A few things: The value would be relatively low to a knowledgable buyer, probaby less than $300 around here. They have a well deserved reputation as the "jamomatic", you can google that word and see more results than you would probably care to read. Remington will no longer repair them due to systemic failures, so you can't send one back to them.
I have owned two, both failed, and failed again after being repaired. Some will tell you the failures can be avoided by keeping the chambers clean, but actually it is a design flaw that cannot be corrected with more maintenance.
The way you tell the difference between the 760 (pump) mag and the 742 (semi-auto) is the 742 will have a small lever near the bottom to depress the magazine follower, allowing the bolt to close.
And yes, the magazines historically have been troublesome. They are made of sheet metal, and after a few years tend to bend and flex out of original shape. Sometimes they shorten, or the front lip wears, and they will not stay in the rifle when fired.
Sorry for the bad news, but google will explain it pretty well.