After typing this, it sounds like I'm kinda being a smart (*) but I really am not meaning for it to sound like that. I truly think you asked 2 good questions.
Well, having owned several businesses, I've done probably almost no market research on any of them and almost all were a success and one made me LOTS os profit. Others could have done better but the way I am, if I start a business, even if it is going well, if I don't like it, I sell it. Roll with it..... market research doesn't really tell ME anything about what I'd be doing. A person can research for 10 years and it won't really determine a thing except that he/she wasted 10 years not making money. I've never really believed in the phrase 'projected sales'. I can sell. It doesn't matter if it is a product or a service, a 'good' salesperson is a salesperson.
The thing that lead me to a/c units was that the warehouse where I buy my washers and dryers from told me about a deal. I looked them up and the store price is just over 3x of what I can buy them for. So... we did it. lol I think we can sell them within a month but even if we have to hold some until next summer, they will eventually sell. Another thing is that I like to have a lot of the same thing to sell. If for example I have 1 a/c unit I can't really advertise it much because once it is sold, I don't have any more. And it wouldn't be worth spending any advertising dollars for just 1 or even 10 units. But if I have 250 of them, I can use all my marketing ability and have many different avenues of selling them. Once I retired from my main business of 19 years I was going to start selling small profit items. Then one day I was watching Craigslist Hunter and he said something that caught my attention. He said I could either sell 20 small profit item at for example $5 profit or 1 big profit item at $100 profit. So I chose the higher profit items.
I am more than open to this post leading in to conversations about selling things.