Building up second hand frames to resell
There could be some sticky points with what you propose in that even though you won't see what you're doing as a business, your customers probably will. You'll be accountable to doing right by them and there will be times when that will feel like work, be work, and cost you money or time like work.
The answer to your question also depends entirely on what you're paying, if anything, for the bikes and parts you're working with. In some times and places, old bikes in need of work that's hard to time-justify outside of being a labor of love are abundant. But if there's a strong used market, maybe not such much.
A very good avenue for the passion you're talking about is getting involved with community or nonprofit bike shops as a volunteer. Most cities have organizations or businesses in this vein, and most of them welcome volunteer labor to make sellable bikes out of their old parts. And, they typically have proper insurance and other trappings of actual businesses in place, which are all other ways the outside world could expect you to be a business even if you don't consider yourself one if you're just doing it on your own.
Some of these community initiatives keep a very low profile, often without any web presence, and might be tricky to find. They are sometimes side projects at various charities (focused on homelessness, substance abuse or prisoner resettlement), schools, churches or even squats. It's definitely worth asking around in local bike shops or clubs.
If you don't have a community bike shop or project nearby, consider starting one. But the time you've done all that asking around you will likely have some contacts etc.