Find a few models of MP3 players you are interested in. Go to ebay and you should see plenty of models for sale.
Each seller has a little number beside his name, this is a feedback score. You can click it and see what other buyers have said about the seller. If the price is right, and you are comfortable with the seller, make a bid.
Beware!!! If it's too good to be true, it often is. No one sells $100.00 MP3 players for 20.00 new in a box, and are legitimate.
I have sold on e-bay as a side business for about 3 years. A few months ago, my account got hacked. I only found out from someone calling about the cell phones I had listed on Ebay. I wasn't selling any cell phones. I checked and sure enough under my name someone was selling very expensive GSM cell phones that listed retail for around 800.00 ea, they were offering 5 phones for 1000.00.And they had around a dozen listings. I managed to get e-bay to de list the items, and get my account back under my control, but was amazed at the number (100's) of people that were e-mailing asking about shipping, sending money, could they just buy 2 phones not 5, etc etc . People out there seem to be very content to get duped out of their money.
I mention all this just to make sure you understand there are plenty of people on e-bay that are legit, but there also are a few that use popular items (MP3 players, cell phones etc) to con people out of a lot of money.
I suggest sending an e-mail to the buyer, if they respond from an e-mail account outside of north America, be cautious.
There is my 2 1/2 cents.