I don't represent RedWeek but I'll try to help you here.
One of the main reasons for charging a membership is to keep a lot of the spammers and scam artists from infiltrating the ads. Those who pay for their membership are showing signs that they are serious about any inquiries.
For instance, you can also advertise on Craigslist for free. But once you do, you will get a flood of responses that are less-than-legitimate.
The $15 yearly fee is very reasonable considering that you are dealing with a timeshare.
Lance C.