Dan has given you great advice--I'll add a little more. As Dan says, most deals are done through Paypal, which means both the buyer & seller need to have an account.
If you want to buy a lens @ the Marketplace, have a Paypal account, but have not made a buy-transaction before, you can give your Paypal e-mail account to the seller, who, in turn, will send a bill to you. Then, you only need to log into Paypal, where you find the bill the selller sent. At this point, you select the amount of payment and send it off to the seller--Paypal does the rest and you get a receipt in your e-mail inbox. Clear as mud?
Good luck.
JT