Most eBay drink coupon offers show the set of 4 all attached, as they come from Southwest or Chase. I can't say that I ever seen an auction that showed 6 separate coupons. Even in the heyday after eBay was flooded with all the BS class action coupons they were still shown as sheets or connected sets. I'd certainly be suspicious of anyone offering individual coupons. Given what you describe, I'd suspect the person selling them works for (or is connected to someone who works for) aircraft cleaning crew, or otherwise somehow has access to drink coupons that have been thrown away. It seems unlikely an FA would take such a risk.