The timeshare or membership RV park business has been changing. Some shrinkage as badly managed properties close, or get bought by someone not wanting to be in the membership business. Some policy changes also, with membership parks opening to the public, and on the other side, selling "zone" memberships in affiliates without park memberships. There has been an upside too, with some big growth in membership luxury resort communities, but that's not the same as memberships with affiliate privileges.
If buying someone's membership, usually steeply discounted, you need to understand just what you are buying: quality of the park, conditions for using it, ongoing costs, affiliation costs and rules. I know one park in my area selling memberships for $200, but annual charges are several times that.