Though I'm not particularly speaking of this instance, some things to keep in mind....
Nothing fishy or off about it...not that you see them all the time, but plenty of females that have locked with males or even gravid females are bought/sold quite often.
As far as the weights, 1,250 grams isn't necessarily too light to be breeding, all depends on the individual snake. As for it being a proven breeder and breeding again at 1,250 grams, it could be 1 of 3 things...
1) The female is NOT a proven breeder
2) Maybe she hasn't regained a great amount of weight and could benefit from not breeding this year.
3) She could just be a small snake...it happens.
Either of the 3 scenarios is entirely plausible, which would logically bring you to this...
Rule of thumb when buying whether it be from Ralph Davis or Joe down the street. Ask questions and be confident in your purchase. If you're not confident, do not buy.