I think most private trackers are trying to avoid developing a reputation like that of IPTorrents [1], which is notorious for allowing people to "donate" for accounts.
Most private trackers also explicitly forbid members from selling invites for money, and, more recently, giving them to strangers.
[1] https://www.reddit.com/r/trackers/comments/4xta11/so_lets_ta...
The higher end trackers are communities and they typically want people who'll fit in. One way to do this (and the one that's employed) is to only let people in if they're vouched for by someone who's already proved themself (get an invite from an existing member) or if they've proved themself on another tracker (get an invite from the tracker itself through the invite forum on another).
Lowering the barrier to entry by letting people simply buy their way in results in the tracker as a whole being degraded. This can be seen through the recent replacements for What.CD. One of the new trackers chose to heavily limit membership and focus on a small group of prolific uploaders that had proven themselves somehow while the other chose to have pretty much open signups. The pickier tracker, despite far more stringent entry requirements and a tiny fraction of the user count, currently has the larger catalogue and higher ratio of FLAC.
Also, my limited experience with pay-to-enter trackers has been that they're pretty much on par with public trackers. Perhaps even lower quality, definitely not close to the What.CDs of the world.
I'm just looking for more 2160p really, but so far no trackers I've found, public or private have satisfied that. Usenet has actually given the best result so far.
Unfortunately, community is what leads to content. There's an insane amount of stuff that's available on private torrent trackers that hasn't ever touched Usenet and the kinds of people with access to it aren't the kinds of people to put it on Usenet.
Unfortunately you get mixed results in terms of how things are labeled on public trackers and I haven't had a private tracker account since what.cd died.
I'll have to try out a few of these torrent search tools and see how it works out, but most private trackers didn't want to just take cash and seemed to want you to jump through hoops instead, so unless I can buy accounts via other means I'll probably just stick to public.