Let's hash this over on a number of levels.
Legality of selling them: Website says no. The T&C on the back of the cheque makes no mention of selling. Slightly gray area but since the website says no, Marriott would be within their legal rights to void the cheques and shut down your elite account due to you selling the cheques.
I put that in there so that you understand maximum downside risk in selling the cheques.

Practicality of selling them: If you put them on ebay, Marriott will get them taken down. You used to be able to find Bonus Bucks/Premium Pounds/Extra Euros on ebay. No more; Marriott aggressively has them removed and they've mostly done away with the program due to abuses.
If you sell them on craigslist or some other less visible website (NOT FT!), you will likely be able to sell them.

Risk of being caught: Pretty miniscule. There is a unique number on all gift cheques so they can be traced back to you if Marriott puts in the effort. However, if you're a serial seller of gift cheques, their usage will likely get red flagged.
In the past I've ordered up gift cheques many, many times. But I always use them for my personal stays; no one but me has ever used a gift cheque issued to me. But due to my high redemptions in a fairly short time period, I bet that Marriott audited their usage and matched them with my hotel bills.

Bottom line for me is that the risk of being caught is small. But the rewards for selling them are also small and the penalty for being caught is just not worth the small risk/reward.