Some years back I sold my TAP-28 bare boards for $10 or three for $25 and sold about 200 of them over the course of a couple years. It was a different time back then. To get board costs to a few dollars
per board, you had to order in large quantities, 100+ boards at a time – getting a small quantity of boards made was around
three boards for
$75! In order to make a sellable board, a lot of effort was put into providing clear, concise documentation, including schematics, board drawings, assembly instructions, etc. I did make a small amount of profit on the boards, but the biggest reason I sold them was to have cheap (relatively) boards for myself.
The situation today is completely different. You can buy double-sided boards for as little as $5 or even $2
for ten boards from any number of Chinese vendors, using free board layout software (EasyEDA recommended). Selling a bare board for even $5 when somebody can make their own for 50 cents a piece may be a tough sell.
If you're contemplating selling assembled boards, first check out what you can buy on eBay and AliExpress. You mentioned power supplies. Can you complete against a module like this, available for less than $10?
View attachment 132989
If you really have got a design for something that's not easily available, you might try tindee.com.