It depends. The nature of a points system is that every purchase is essentially a peak season purchase in the eyes of the buyer. For Marriott it does depend on the resort and season. Marriott has ROFR on some properties and not on others, while 60% is a good rough guideline, it only matters if you get a legit offer and for things like Grande Vista, that is not likely to happen at 60% of even the preconstruction prices. They will not automatically buy it back. OTOH, you have the option of buying in to many properties at 60% or LESS so you can use this difference to your advantage. As with anything else you buy and consider selling at some point, the money is made on the front side with the purchase. It is my opinion that those buying in to DVC now are very unlikely to make money on their purchases like many have in the past. It is likely a well bought Marriott purchase will do better than a comparable DVC purchase in terms of long term $$$ value even without taking into account yearly savings. It is also likely a DVC retail purchase will do better than a Marriott retail purchase other than for a 2042 resort contract.
To exchange Marriott you currently must below to II at a cost of around $84 per year and exchanges to other Marriott's are usually $99 (it went up, no longer $89). You get limited request first to non Marriott's and full request first to other Marriott's compared to DVC. But you get the potential of a bonus week, usually much lower fees overall, access to getaways and exchanges online; all of which are a better value for many than avoiding an yearly membership fee. There are independent exchange companies though that offer many of these options cheaper and/or better for certain situations.
I would caution people (DVC, Marriott or otherwise) from putting too much stock in the info provided by a timeshare sales person, it's not a good choice to do so. Even when well intentioned, they are often not very accurate in their info.