My females have usually hit their breeding peak between the ages of 6-8. They carry on breeding safely after that - when I've tried it, they've produced eggs well into their teens and I retired them when they were too slow to regain condition after laying.
However, after about the age of 8, the sizes of the clutches start to drop and the proportion of infertile eggs increases. This may be what you're seeing with the female - or the male could've "run out of steam".
At 10-12 they're certainly past their prime. I wouldn't expect to pay a premium for them as a "breeding pair", even if they had been producing successful clutches up til now. The fact that the last two clutches were infertile could well indicate that as a pair, they're probably more suited to a nice retirement home!
In your shoes, I wouldn't buy them for breeding. Better to save your cash for a much younger pair.