Secondhand Apple hardware prices are pretty ridiculous. Personally, i take a much more conservative valuation on computer hardware in general - similar to value for tax purposes.
Depreciate the value over 5 years to zero. 2.5 years old = roughly half price, if that.
i.e., a 5 year old computer is worth pretty much close to zero dollars. Paying 700 for one is nuts.
You don't want to buy someone else problems, there's a reason it is for sale.
If its working and a 13" 2011 model i'd pay a couple of hundred for it, no more, and even that is being generous IMHO.
Others here will probably disagree, but machines that old really are past their best before date in terms of battery, external connectivity (no HDMI, no USB3 for example) and well out of warranty. There's a very real risk it will break and if you paid 700 bucks for it you're left with a very expensive door-stop.
edit:
Don't get me wrong, I'm not just going to throw a 5 year old machine in the bin if i bought it new and it still works. But i'm certainly not going to buy a 5 year old machine for any sort of real money, either.