You got it backwards. A) The seller gets the same price they agreed to-- no harm at all; B) Offers won't go down, they will go up. Supply will be reduced and buyers will fear their low numbers will get stolen by Marriott; C) Low priced comps will disappear from the market (shifting leverage to the seller) and providing current owners with support for their developer priced assets. Think about it. If you bought at $25K, you'd feel a whole lot better about your investment if the re-sales on eBay were going for $15K than $8K.
Although I believe Marriott is only doing this when they are certain it is a profit generating transaction, you can be sure they understand the secondary market. It only helps their developer sales when they can show that value is retained or at least not going to zero.