your question seems more related to transfers to people not on your account. it seems to be a fairly common practice in this forum, but those who do it do not discuss the sales aspect, if that is why they are doing it.

discussing sales of points/miles is not allowed on FT.

i'm sure AMEX includes restrictions in its fine print, like almost all airlines/hotel programs do, not just regarding sales, but transferring into accounts of people who are not on the card account.

but in practice i do not recall anyone saying that the miles were ever deducted or account penalized in any way.