people are buying and selling anything and everything, the bigger the item the more expensive the postage so best bargains on smaller items.
Paypal is a company that you set an account up with who handles your financial transactions. Instead of using your credit card number with each individual transaction you only give paypal your details and all transactions are done through them on a secure site.
Feedback is when youve received your item - you go back and leave the seller feedback saying how good/bad they were to deal with, basically you rate the transaction.