UK you can be a sole trader, basically a person acting as a business. I believe there are some rules about how you legally present yourself (like in your contracts your name has to be "Bob Smith trading as Acme Widgets").<p><a href="https://www.gov.uk/set-up-sole-trader" rel="nofollow">https://www.gov.uk/set-up-sole-trader</a><p>However, as the page says, you're personally liable if something went seriously wrong.<p>The alternative is to pay £50 to register as a ltd company, though there are costs in running one (£12.50 per year + doing your own accounts vs paying an accountant £200-£300).
In the US you can use a fictitious name dba "doing business as" name. You can also open up a bank account using a dba name. You may need to file taxes as a sole proprietor.<p><a href="https://www.sba.gov/starting-business/choose-register-your-business/register-your-business-name" rel="nofollow">https://www.sba.gov/starting-business/choose-register-your-b...</a><p>I think this is called a "trade name" in other countries. Wikipedia has some examples for other countries.<p><a href="https://en.wikipedia.org/wiki/Trade_name" rel="nofollow">https://en.wikipedia.org/wiki/Trade_name</a>
In the Netherlands, there is a threshold, and you do not have to incorporate if you stay under that, and you'll still have to report it. But this obviously depends on the country you're in.<p>Paypal could be a good way to take money from people privately, and it's probably entirely legal, up to a certain point.
Not sure how general this is, but in Poland if you have SaaS you need to have a company (because it's a "service"). But when you are selling just source code to someone else, it can be counted as personal income (no company needed).