In any case, the thing about making matter into exactly what you want, is that you can make really good art out of it - you can shape matter as you please, thus, sculpture. Or a painting. You can take raw materials, and come out with a project that's a lot more valuable than you started with (as art does),and if in your case, the original materials weren't needed, who is actually gonna know?
You can make originals, easy, just by making something out of your head, tweaking to your own tastes, and selling as part of a series or as reproductions. You can make reproductions - note that forgery will get you in trouble, but openly labeling your products as reproductions (and possibly adding or altering some detail, so it is obviously not forgery) will let you escape a lot of trouble - and people are willing to pay a lot, especially since you can go from "simple replica" to "museum-quality" reproductions, with accompanying price ranges, pretty easily. You can get your hands on reproductions by purchasing them, or visit museums and libraries for access to originals (once you've built up a reputation, you may be able to arrange for closer visits as an expert).
You should think about your medium - wood sculpture, you can buy a few cords of wood and have a bonfire every day, and no one knows your stash of wood carvings didn't come from that wood. Paint and canvases are also pretty cheap, you can easily find something to do with them (even if it is raw material for your talent, even if it is a side hobby, even if it is hide or discard them somehow). It would be wise to pick up, as a hobby, whatever "art" you choose to sell (or information on the historical techniques replicated, whatever), since people may be asking about technique... on the other hand, artists are supposed to be eccentric, so you just need to have something to say, or never talk to people about art, or claim your best work comes in trances or something.
Along the same lines, you might take up jewelry making. Even working with basic materials, copper or aluminum wires or something like that, the ability to shape precisely will let you form salable pieces - and since the value is the effort (or supposed effort, though having secret matter-bending talent isn't exactly less effort than making art the usual way), it won't matter that your original materials are inexpensive, or that your tools are basic. You can work you way up to grander, more expensive works - and higher prices - without anyone questioning where the pieces are coming from.
Buy raw materials, use some of them to transmute with, use some to just play with, give some away, have a workshop or three stuffed with odd bits and pieces, claim you often trade raw materials for finished products and that's why you rarely are seen purchasing, whatever. The thing is, there's no cheat - because the stuff is real, the delicate shaping is what people pay for, so there's no real incentive for people to fake it.
Someone else has already mentioned pawn shops and antiques. Just, it's a really good cover for all sorts of knickknacks and oddball items, especially if you use your ability to copy, or repair, or supplement genuine finds.
Also, for seed money - you can probably create a hefty chunk of cash with a one-time creation of precious metal jewelry - plain chains or bracelets or earrings in gold and silver, maybe different thickness and styles. A one time sale, or actually selling one or two at a time from an obviously limited stock, can be explained away as an inheritance, or gifts, or whatnot - especially since you'll only need seed money for raw materials at this point, so you can keep the amount low and make your major money in art, and build back up to precious metal when you have the cash flow to cover the difference in your raw materials and outputs.
Also, as a side note, I'm assuming that the power isn't to create matter from nothing (that seems a bit too universe breaking, not to mention dangerous), but rather to transmute one kind of matter into another - so it is possible to take up a block of wood, and shape it to a sculpture (or at least it's easier than trying to form it from nothing, which might take mass from air, or extra-extra energy, or something). Picking up a stone, and changing it to gold, or a book, or some bread, is quite universe-breaking enough. So I am assuming most of the raw materials are used up as, well, raw materials.