So, if I understand right - there'd be the regular T11, then a super secret area for only the most magical of us all (coin in bottle? more like coin in YOUR SKULL), where T11 would sell a special collection of tricks? Only the members of that secret area could get those tricks. Then it'd be truly underground.
Firstly, if there's a question, Google has the answer. That's just the truth. Google's superpower is being able to find ANYTHING you search for. Type in, How many times did robotsunshine pee today, and you'll get an answer (fun fact: 4 times!). Do an image search and... well, don't image search that.
If someone wants to market an effect, such an avenue doesn't help them. Let's Secret-T11 is created. By its nature, it'd only accept people on the T11 forum, already a relatively small populace. Then, we're handpicking about 1/4 of everyone for Secret-T11. We'd have like 20 people - and probably only 14 or 15 who'd buy the Secret-Tricks. Then, what if it's a card trick, and Johnny McShadowcoins isn't interested? Probably only 1/2 to 3/4 would buy each trick. And even if the Secret-group were larger, it's still a tiny fraction of people available for money snatchin'. So, how many magicians, ready and eager to market their new Swallowing the Moon effect, would say, I'll pick that tiny audience subset on Theory 11!, when they could just sell it on the regular site for everyone to buy?
Honestly, if you want real underground stuff, invent it and share with friends (magician friends, of course!). As long as millions of people around the world with a Visa card can get your trick, it's not going to be underground.