A couple of points:

(1) Ebay does not respect your privacy and gladly provides your personal info (for buyers, sellers, bidders) to hotels and airlines, and probably other companies. I don't even think ebay even requires a subpoena as the info is provided under a ridiculous broad interpretation of the "VERO program"

(2) If the award does not say "illegal to sell" then I don't see how Marriott or SPG can threaten you with anything. However, my guess is that the bonus bucks and SPG 50s are awarded through the respective point programs, and the terms of those programs clearly prohibit sale or barter of awards (which would include these items). They also might have a fraud claim to the extent you fail to disclose to the buyer that the item is void and worthless if sold though the buyer would have a better claim.

As for the morality of selling these items, well, just ask everyone who uses coupon connnection But seriously every program from foodstamps to miles that has tried to prohibit people from transferring coupons for cash has failed. It is 100% predictable that people will sell something they don't value to someone who does value it - the law of economics.

I do find it quite entertaining that SPG and Marriott (like Delta and AA) would waste time and resources tracking down each of these sellers. But not as pathetic as all the music companies trying to sue teenagers for illegal downloads