I could not find the 52 tooth chain ring I wanted for my old Oxygen Emate during lock down and bought a cheap narrow wide one off ebay, just to have one if I needed to change it. You can run a rear hub bike many miles and in my case sometimes years beyond when you should have changed the cassette, chain ring and chain and I carried on using it and have only now put a new transmission on.
I spoiled my old Oxygen Emate with a new cassette, chain ring and chain. I put on the cheap one I had bought during lock down. It may bed in as it is probably made of soft alloy, but it really is not very good. I will ride it for a while to see whether it wears in!
I also recently bought a 36 volt 15Ah battery from Yose for £165.62 delivered including a charger, and after using it on a few shopping trips now, it feels a very strong resilient battery. From my perspective a bargain.
With my crank drive bike when I was commuting to work cross country and then riding it home on the road fast the dive train took a pounding and my top gear with the least teeth (12) would wear out before the rest of the cassette. I found cheap copy replacement top gear cogs to extend the life of my cassette and would change the chain on the wear limit and at the same time put one of these cheap top gear cogs on to replace the worn one. They worked really well.
Now I have retired I have just lowered the overall gearing on that bike by replacing the 42 tooth chain ring with a 38 tooth one, and bought an 11 tooth copy replacement top gear cog to allow me to occasionally ride at higher speed on the road on the rare occasion I might want or need to. I bought the cog from the same company and it would not fit, it was obviously too wide.
So you are right, buying parts of the internet, you can find cheap parts can be remarkably good and expensive parts not as good as you expected, so like you I will sometimes buy a cheaper bit to find out!