I agree with pjlla - ebay won't be fast enough for you.
I'd say a yard sale would probably be the quickest, but to have one tomorrow you would need to get it in the paper now so it is advertised tomorrow morning. I'd make it a two day yard sale. If you have some larger pieces of furniture you no longer care for, this is the place to sell them - put them on the lawn at your sale. I am looking at a couch right now that I don't like so I will probably yard sale it this year and I should be able to get $100 for it - it's in great shape, it's just too small for my liking.
Pawning is the quickest way, but you have to be prepared to pay high interest ($300 you'd probably have to pay back $400 in 30 days, can you really do that?). You could just sell the pawn shop stuff you don't want back - old VHS movies, DVD's, any working gadgets that you never use, etc.
I realize also that not everyone can do what we do, but my BF and I are very good at finding antiques, vintage guitars, vintage amplifiers, etc. and reselling them. So to answer your question, if we had to raise $300 in 10 days, we'd probably just take one of the many instruments we have and turn it for a quick profit.