When I use Paypal for eBay stuff, I have the funds pulled from my checking account. I think you can also have it pull from your credit card, so then you'd have your normal interest terms for a credit card purchase.
And last time I had a personal account they allowed you about 6 transactions, per month, for free. After that they will hold or freeze it for the rest of the month and you start all over again the following month. If you do more than 6 transactions (I believe that number is correct) on a personal account, they suggest you upgrade to a business or premium account and the transactions are unlimited, but then of course there's a fee for each transaction depending on the amount.
I do believe, and may be totally wrong, that if someone pays you through paypal with their checking acct, the fee will be zilch to minimal for you (depending on the amount), but if they pay you through paypal using a credit card, paypal takes more of a percentage of the monies. Paypal never costs the buyer a dime...only you that are receiving the payment.
I have seen a difference in percents that come out of the paypal monies I've received through ebay auctions, and I'm assuming that's because some paid with a credit card and some an instant transfer with their checking account, I never really thought about it much until I had a somewhat high sale on ebay. I sold something for $1800.00...the guy paid me through paypal and I only ended up getting around $1550 (and that did not count the auction listing fee either, just paypal). In the future I will not take an amount over $500 on paypal, the fees were just too high. .a couple houndred dollars ok, but anything over you are really going to loose some cash with the fees.
The upside to using paypal is that someone who wants to use their credit card can pay you, you will pay the fees, but sometimes maybe loosing alittle on fees is worth the sale...you have to weigh that and decide for yourself.