Hi yaamba,
Flippa.com (Link hidden: Login to view) is a great place to buy websites including established e-Commerce websites.
You generally do pay a premium when you buy established websites though - there are people out there who make a living from building up websites then reselling them.
For this reason, I think it's worth you at least considering setting up your own website.
We've recently released the Beta version of SaleHoo Stores which means you can setup your own e-Commerce website very easily and very cheaply (for $97 for the entire website including shopping cart and this also covers your first year's hosting fee).
The idea behind SaleHoo Stores was to help people just like you who want to own a store but would rather not have to deal with the time and energy involved with starting a store from scratch.
Really the only difference between buying an established site and creating your own one with SaleHoo Stores is that the cost (SaleHoo Stores will be a lot cheaper!) and the fact that you will have to purchase your own wholesale items and list them on your wesbite - which is really easy ;)
If you want to read more about SaleHoo Stores, check out this post I wrote about it last week:
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If you do choose to go down the road of buying an established website, make sure you check whether the store includes any inventory and the cost of getting that inventory shipped to you (some stores have 'dummy' items listed but do not actually set you up with items to sell and some do include items but it might be very costly to ship the items to you).
Also ask the seller how the site gets traffic.
If it ranks well in Google for terms like "baby health products" or "baby boy pajamas" then you can expect to get some consistent traffic to your site.
Be realistic with expectations though, it is unlikely that the site will rank on the first page for highly targeted keywords like "baby clothing" which are very competitive in Google.
If the website comes with an active Facebook or Twitter page, then this is great too as it can mean you have a targeted, loyal customer base.
Beware of any data that the website seller shows you such as monthly visits to your website - I have heard of website flippers who pay for large amounts of traffic to go to their websites before they sell them to make them look more popular than they are.
Here is that link again to SaleHoo Stores for you to check out:
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Good luck and let us know how you get on!