Fag packet numbers: you could buy a similar amount from BA or Starwood (assuming sufficient members of HHA etc) for an amount of miles in the same ballpark - assuming you pay with Amex Premium jobbie.

Yes, it may be a gnat's more expensive but it's risk free - I'd want a considerably greater ROI to take the chance that (a) the seller is legit as well as (b) running the risk of having the BAEC acc nuked...

Caveat emptor!