I know some people who actively trade websites for profits.
One of the better places to buy website is Flippa.com as @antropy recommended.
However, you should verify the stats of the website properly before going ahead and clicking the buy button.
Look for stats on SEMrush kind of website.
Also, keep in mind that the domain name separately holds some value. So, look for websites which have good domain names as well. Then, you'd have an easier time on encashing those either in terms of money or value, ranking etc.
For domain names, I'd suggest having a look at NamePros community.
Let me know if you have any other doubts. I'd be happy to help.
@Arpit Agrawal has a very good point. Make sure you get what you pay for, that the customer base actually exists. We have had several people come to us after buying an e-commerce business for £xk for us to have to inform them that the data on the website, customer and orders, analytics etc.. is bogus.
Shopify exchange could be one. I have not used it but in any case some due dilligence into where the problem is and why it is for sale in the first place. The problem in most cases would properly be the lack of a marketing and advertising plan that works.