Ok, I suppose you have already got a design for you ecommerce engine, hosted it successfully, submitted to search engines and exchanged a couple of good links. Now its high time to think of getting cash from your clients. I prefer and recomment 2 ways of doing it.
First one is PayPal, its probably the fastest, the most secure and the cheapest way to get money from client. Its very convenient if you live in EU or US. This payment system works great both with US and Euro banks. You can withdraw money at your bank account right after your identity verification.
The problem is that PayPal does not work with all countries and territories, so if you live in a country not supported by this payment gateway you will not be able to register and work using it. As an alternative we would advise 2CheckOut which may be an official retailer of your goods. Its a bit more expensive, not so secure, charges for chargebacks are higher and in general not as convenient as PayPal, but it supports much more countries and works with all credit cards.
Such old and classic methods of money transfer as Wire Transfer and Western Union are quite expensive, Moneybookers frankly sucks, CronoPay has not quite clear reputation as for me.
Well, I must say that there are other payment gateways billing credit cards and paypal accounts, but I have no experience working with them and can say nothing about it.