No idea, I've only ever subscribed to a single forum over the years and that was a Vauxhall Vectra one as it meant I could see a lot of the How To stuff and very hand it was as well. On XRV.org it used to be you couldn't sell stuff on there unless you were a subscriber (or even post on that forum) and there was a section with discount codes I think.
At some point during my membership of UKGSer forum they switched to a subscription model and now I can't see a lot of the threads including the For Sale section. This is a bit weird because that means paying members only get other paying members making offers on stuff?? But as I only really read it to see the BMW mechanical failures constantly coming up, there is enough content to keep my black heart happy. It's similar to my love of problems pages, love the problems, have no interest in the answers.