I, too, have bought and sold for many years. I did it as a way to upgrade boats without paying much.
I had a Grady White I was making payments on. I sold it, and had $4k left over after payoff. I realized I wanted a boat, but didn't want to get into debt again. So I looked around and found a $1,100 Bayliner Trophy on Craigslist. Did transom repairs ($600) and upgraded the motor ($2,000) and ran it for a year. Clean boat, but I hated the ride. Sold it for $5,500, giving an $1,800 profit. Not a bad start! I put the $4k back in the bank. I not had an $1,800 boat budget.
Than I found a $400 runabout on Craigslist with a nice 90 HP on the back. I kept the motor, stripped the boat for parts and dumped it for free, and sold the trailer for $550, ($100 profit plus a ton of good stuff) Then I found a sweet 17 Aquasport with no motor for $1,200 and I put that motor on it. That rig cost me a net of $1,100 of my $1,800 - nice! I was still $700 ahead. I ran that boat for the next 2 years. GREAT little boat, by the way.
I sold that boat for $4,500 and and bought my current 22' Offshore center console bay boat for $5,500. Cost me $300 net, minus the sweat equity. I still have that boat 12 years later, and it's still awesome. SO worth it, and I got to have fun fishing the whole time.
Since then, I have bought and sold many, many boats.
Like you, WeeHooker, my wife complained a little at first. But then she started supporting me. She would track the income and expenditures, keep the books balanced and make sure the taxes were paid. I hate paperwork, so it was really nice having everything handled. This was our primary source of income for years, along with boat repair work, while taking care of sick parents.
Now that the parents are gone, I'm scaling back on buying and selling and going back to construction work. I enjoy it much more. Tired of fixing boats and motors for cheap, whiny customers. My home improvement customers are always thrilled when I'm done.
I still have some inventory left to sell- 3 boats, 5 trailers and about 20 outboard motors, give or take.
My awesome Princecraft 16 is supposed to sell tomorrow. I don't really want to sell that boat, but I have two 16' tin boats, so one has to go. It's a great little boat. Hopefully, the new buyer loves it.
Then I have a 19' CC that needs a floor. I'll probably do that in the spring.
Finally, I have a pontoon that needs a floor, and I'll probably sell that in the spring too. At that point, I'll be out of the boat selling business, at least for awhile. We'll see what the future holds.