It is a matter of public record what the house last sold for. So you can find that out, plus figure in any improvements that they may've made.
I'd remember this - if they bought the house anymore than 3/4 years ago they bought when prices were inflated so they might not be able to accept any less than the list amount, or close to it.
We just moved into our house; it was an estate sale - the man who lived in it bought it 20 years ago; We got it for 50K less than the original list price
I'm a firm believer however that you don't know your sellers situation, this is a business not personal transaction so I wouldn't care if I low balled and they thought my offer was "insulting" - I'm trying to get the best bang for my buck.
Good luck!