My brother does a lot of builds for people as a side hustle. He draws out the plans for them and the supplies needed and then just tosses it in a file. I mentioned to him that he should try offering those plans to people online that want to do DIY projects. A lot of what he builds are fences, gates, decks, sheds, and greenhouses. I feel like he could charge $3 to $5 for these and make decent passive money. I mentioned the idea to him and he said he can't be bothered doing all that but I can if I want to. So I am thinking of trying it.

That being said, I am not sure if I should use my own site to sell them or use an already established webstore. What do you guys think?

That is actually a really solid idea you have there! I think the best approach would be to start somewhere like Etsy. That may be the best platform for it as they offer digital sales and they don't take as high of a percentage as other places. If you gain some traction, you can start adding tags to the digital blueprints that tell people to go to your own website and offer special blueprints there that aren't on Etsy.

I agree about making an Etsy account and selling them there first. Etsy will promote it for you as well for free but if you wanted to boost it, there are paid options on there too which could come in handy for you.

Does eBay or Amazon allow for digital sales by chance? If so, you can look into options on those platforms too. I know both used to allow it but I am not sure if they still do.

Creative Market might be another option. I know they are a digital asset sort of shop. My sister uses them for stuff she prints out.

I don't know if they cover stuff like digital plans for DIY stuff or not but you can look into it.