Buying an invite for TL is not a smart idea, they have regular open signups. You put all your accounts at risk for little gain.
Previously, when I was really into torrenting I climbed the ladder really well, I was in the forum sections where staff would share the details of banned users. They mostly cared about cheaters, unless it was a small site trying to be exclusive. I knew people who would go to tracker staff and out people for trading and selling and nothing would happen.
But overall if you want to get into the torrent community buying and trading isn't worth it. But if you just want a single solid torrent site and are willing to pay TL is the one to do it with.
IPT is solid from my memory so there won't be many that are better as a general torrent site. But for niche obivously there are tons.
But since you're already in a torrent community, Check out the IPT forums back in my day they had recruitment threads in the forums. Hang out on their chat and be friendly, don't ask for invites just be friendly after a while people will start offering invites especially if you say you aren't there yet.
Remember that user invites carry the risk that if someone in your invite tree breaks a golden rule (trading/selling invites) you will be banned too. Always prefer official recruitement.