Amazon's fulfilment system co-mingles. Checking the seller is pointless, it is an accounting thing only. If it's fulfilled by amazon, then the source can come from _any_ FBA merchant.
A clear indicator that it's directly from Amazon and not intermingled is "Ships from and sold by Amazon". Any mention of fulfillment in that text should make you slightly more cautious.
Obviously, this entire situation is stupid and they shouldn't force you to parse their little blurbs about selling/fulfillment, but the information is there.