I buy thing from all kinds of B2C shopping mall in china, like amazon.cn, 360buy, dangdang.com, also C2C like taobao.
they all have something in common, they all need an account to register, and then login.
We’re making our own shopping mall, and usual we have the account login and registration, but mike said US people don’t likw to register a account, they want to pay fast and directly, save time, and so did andrew. So we decide to carry out the surgery for our shopping cart system, luckily it’s zencart.
Integrated Checkout Without Account
When a visitor adds something to their cart and proceeds to checkout, they are greeted with three options. They can sign in to their existing account, they can create a new account, or they can proceed through checkout without an account. They are only given this third option if they have something in their cart. After entering their billing information they are sent to checkout_shipping. If they somehow manage to enter the COWOA sequence without something in their cart, then after they enter their billing information they are directed to their empty cart.
There is a COWOA Support Thread here
Download COWOA for Zencart 1.3.8

I have actually used this function for our own and it is working great. So if you are using ZenCart, and need this type of functionality then use this. It will work great for you! Just make sure to follow the instructions carefully!

January 14th, 2010 at 11:01 am
COWOA is ok, but a bit outdated. ZenMagick (http://www.zenmagick.org/) also supports guest checkout and offers heaps of other advantages.
January 15th, 2010 at 5:45 pm
@DerManoMann tks, yea, your recommendation is right, I’ll try zengagick later
February 5th, 2010 at 6:51 pm
Great mod loves it thanks allot mate,
but one thing if you add to this mod a captcha it will be awesome,
February 5th, 2010 at 10:40 pm
@Ali yea, one day I guess there will be the captcha