If it has been properly lubricated and if it has not been mechanically damaged, then it can last a lifetime. I think the real question in your situation is whether you actually have bearing wear, not a theoretical question as to how long it lasts. If it's worn, it's worn; if it's not, it's not. I wouldn't discount the price just because it "might" have wear or because it's something somebody said to "watch for". It's just a canny purchaser trying to beat you down on price a bit. The age of the unit, and therefore normal wear and tear, should already be reflected in your asking price. It doesn't need another price discount on the same point under a different guise. I would discount the price further from fair market value only if it is being specifically sold as a damaged item "for parts only".