It actually makes sense somewhat for him to sign a bunch of stuff for free.
The more stuff out there, the less all the stuff is worth. So if he floods the market with valid memorabilia, those people will make less and less from what they have.
The trick with the NCAA is that they really do have to prove he accepted money. And if they rule him inelilgble for the year, he will likely immediately join the OBannion lawsuit and the NCAA would be in deep doo doo. With Manziel, they can show how much money the NCAA is making and keeping from athletes and it's a high probability they lose the lawsuit and millions if not billions of dollars.
The NCAA is probably going to tread lightly with Manziel in all likelyhood. Don't be surprised if A&M offers a 2 game suspension plus Manziel paying back the money (assuming he did it) and the NCAA accepting it.