Assuming you are talking about the FN sporter, not a modified military rifle, I can say that they are good rifles and accurate.
But they were a common sporting rifle in that period, sold under several brand names, including Sears. Unless it is the presentation grade (engraved and with select wood and checkering), it is not worth $900, even in new condition. New condition ones go for around $650; in average used condition (85%), they run about $450. Ones with other names go for $100 or so less.
Jim