I sort of disagree I sell or gift all my old Apple products to friends and family, I give them very cheaply or free and they know because it's me that they are well looked after. If something goes wrong then they aren't much out of pocket and they know it was not my doing.
It never has though, all my old Apple products bought between 2010 and 2014 are still going strong, 2010 MacBook Pro (£300 (to my brother who only ever gave me £100, ah well that's little bros for you) in 2013) 2 X iPad minis (100 pound for one free for another) when the next gen came out) an iPhone 4 (£100) and an iPhone 5 (free).
It's a pretty good bet that it'll last a year or two at least for your iMac so price it accordingly.