is there a marketplace for 3d pen drawings?

Art of any kind is difficult to sell. If your "3D drawings" strike a responsive chord with buyers that makes them need to have them, then sure - I suppose there might be a market for them. But getting in touch with that market might not be easy. Any opportunity to show your art to people will generally cost money, sometimes quite a bit. On the low end, you can pay to put pictures up on a website, where they will share space with innumerable others. Perhaps someone will wade through all that, fall in love with your work, and send you money. But that doesn't happen too often. On the high end, you can purchase a booth in a major art fair, for thousands of dollars plus expenses, which can be considerable. You might find buyers that way, but you could just as easily go home without selling anything. I've never heard of a marketplace devoted to plastic doodles, but I suppose you could start one.

If you're worried about compromising your Intellectual Property by posting pictures of drawings which are, by default, copyrighted as soon as they're produced, register the copyrights before you post the images. It's doubtful that anyone will actually try to copy your 3Doodler pictures, but if they do the registration would help you in court.

Andrew Werby
Juxtamorph.com