I used to work for Amazon. Be very careful with Marketplace transaction as if it goes wrong Amazon don't want to know. The feedback system is a bit like eBay's, so check out the feedback before buying and you should be ok.
I've brought a few low cost items, and sold a few things too. The postage is extra as it's not Amazon who send it out, but the individual seller or company. Amazon charge the buyer, then take thier fees. Every so often Amazon transfer money fropm my sales to my bank account. I prefer selling some things on there to ebay as you state the price you want, and it lasts for 3 months or so (before requiring a relisting). Things like books in good condition are more likely to sell through amazon than ebay anyway. If Amazon sell it, undercut them and get some trade. Also good if you want to sell something that Amazon say will take a few weeks.
As always, buyer beware, I'd read the comments of any seller unless the item is low enough cost that you can afford not to recieve the item.