Buying an online business is in fashion these days. One of the striking features of some online businesses is, it is asset light. An online business may or may not have an offline presence and it's very attractive for many buyers. Online businesses strive on one of the verticals - Software tools(SaaS), marketplace, content based and games etc. Many of them rely on advertising for their revenues, while some of them may charge for the product itself. What attracts them the most is, their ability to run in a streamlined manner. Several online businesses fetch good revenues without any dedicated effort.
You can visit this page - https://www.smergers.com/businesses/internet-businesses-for-sale/s1312/c0/t2/ to buy online business. You can connect with the business owner to get additional information about the business. If you are planning to buy an online business in a sector you understand well, then you can evaluate it. If the online business is targeting consumers, you can do the due diligence yourself to a certain extent.
It is also important to understand what technical stack is used to build the online business and it is something you are comfortable maintaining it in the future.