Testing and safety requirements for selling homemade electricals
For compliance to EU requirements and to carry the 'CE' marking you can self-certify without using a testing agency, and to do so you need to create a declaration of conformity, which authorities can request at any time, and maintain a technical file which documents your evaluation of the design and manufacturing process' ability to maintain compliance - so that'd include some quality plan to periodically check production capabilities, and individual item testing if justified. It's up to you to determine all the requirements that are relevant to your particular application.

I'd follow suggestions to use an off-the-shelf power supply that someone else has assumed the responsibility and liability risk for, and maybe just add in some further conditioning and filtering if you're using if for a low noise application. One big reason for this is that demonstrating EMC compliance might be expensive in itself, and an off-the-shelf PSU will likely have the input suppression sorted for compliance.