I guess there are simply not enough Mac users in South Africa? By all accounts, resale values are very good overseas... Trawling through US or UK Mac forums, and you will regularly find users talking about how little they lost on their sale... I think that might be the case with rapid-turnovers... It appears to be quite common for users to hang-on to their Mac for 1-2 years, before selling... That way they avoid competing with too many model updates...
Personally, I would consider upgrading to a more powerful MBP at the right price, but would only be willing to go back to 2011... No real reason for that, just kind of a mental line in the sand, given that my MBP is a mid-2012 and is still purring along... for a Mac Pro/Mini/iMac, I would be willing to look at 2010 models even, but not much before then...
Would I do the same with a Windows machine? No. I would never consider getting a 2nd hand laptop/pc that is not an Apple. Arguments for and against that view, but that's my take on things....