Eight maybe a bit young for the programming, but he could learn it with help from an adult. The FLL robotics league that uses the NXT robots starts with 4th grade (9-10 years old). And at that age they are expected to do the programming/building by themselves. My son has been doing his own programming since the summer between third and fourth grade without any help from us.... but he did work with a friend and they learned it together using the friend's NXT. Robolab is the easier programming system of the ones out there.
You know your child best.... if he is bright and loves computers and legos, it might be a good fit for him. But if he is easily frustrated by learning new things like that, it might be best to wait.
My suggestion is to buy the kits new from
Lego..... they are pretty pricey on Ebay and such, even used.... and honestly they don't last forever. But Lego is good about helping if there are issues.................P