Speaker life expectancy: How long can you buy used

I'm in the process of assembling a cost-effective second system with high quality used or demo gear for use in a relatively small room (appr. 20qm, normal height, wardrobe, big window, carpet, sofa, apart from the size of the window quite average really).
I saw a pair of Avalon Monitors, that I think could fit the bill at 30% of the original price, mint by the looks of them BUT seven years old! In spite of the good things I hear about Avalon's longevity, I do wonder whether it is wise to buy a seven year used speaker, any speaker.
Apart from considerations of wear, where would you draw the line that even a steeply discounted speaker of justified fame in its own time will likely not be able to compete anymore with new products that may cost just about the same amount of money, new that is. And are there exceptions to the rule?
Any considerations or experience you may want to share?
Karel