For a 1/2 cup of material, your check sizes could be a lot too small, but without educating yourself, you've really got no clue.
I may be able to share with you a buyer that handles a LOT of dental, but I'm honestly not sure as you aren't an established member and security concerns are always huge. I don't have a PO box yet, though I need to get one, and I'm not sure if he does, and I'm not going to hand out his physical address or even his company name, as he's made it a point to maintain limited web presence....anyway.
Remember that ambiguity is almost always the knowledgeable buyers friend, but that it shouldn't be a license to rip off the seller...it's just another service that they can charge for. With that, you shouldn't expect prime prices on white dental material, without doing testing on your part to ensure that it is precious. BUT...even without sorting, you should be able to find an honest buyer, but the only way you'll know they are honest is with sorting.
So, here's my advice. Keep your gold teeth in a separate lot.
White dental material...hit it with a hammer. If it breaks or doesn't move, and you hit it hard enough to make it move, put it in one pile...good chance it's junk. If it smashes, or distorts, put it in another pile...great chance it's precious metal.
Now you've got three different lots, at least the yellow gold and the white malleable material have value, and almost always good value...the non-malleable, is likely junk, but let an expert on dental material make that decision.