and why
Not enough information to answer this question properly, how can you expect a decent answer with such a crap question?
It all depend on what you sell
There are some websites which offer selling your products for free as well as paid
ebay and amazon are the best sites you can trust
and also you can go for classified sites which offer free ads for your products
These are the two best sources to sell the products online
Depends on what you will sell and your goal.
Good sites to sell are ebay or craigslist.
If you are selling many items and have the time and money build an ecommerce website.
ebay or Craig's list.
craigslist is great, very simple easy to use.
or should I create my own website?
and why