Paypal works the best as others have stated. Im assuming you dont understand how paypal works when you dont have the convenience of ebay basically sending the invoice to paypal for you (the whole checkout procedure thing). You can do it one of 2 ways using paypal. So lets say someone needs to pay you...
1. You can send a request for money. Meaning you are selling them something and you would like to send them a bill/reminder to pay, you just send a "request for money/funds" and that person can just hit "pay" or whatever.
2. The person who is trying to pay you, can just go ahead and pay you. For example, if your paypal account handle is
johndoe@northwoods.com, I could go and just blindly send money to
johndoe@northwoods.com.
Either way, you have an electronic copy of the transaction. If you use feature number 1 from above, it leaves some open fields where you can describe what you are asking for funds for. Kind of like a receipt.