There are a lot of free templates out there so you'd have to offer something unique that people would be willing to pay for, like high ctr (click thru' rate) for instance and once a template is sold you'd find it would be shared anyway so it could be had for free before too long. New templates are being made for free and tbh aren't that hard to make. I have a free template site already (in my sig) and the reason I haven't added to it in a while is because some people change or remove links, against license terms, and use for themselves. When I get more time I will add to it but I may just encode everything to be sure I won't get ripped off again. Paid templates are OK if there's a market but free seems to be the norm these days, and is a decent way to get some backlinks as well (my original plan). You or anyone else are welcome to add your templates to it of course, it could do with some 'fresh blood'
I have (and still do) use different new themes but I use the old ones too because I know they'll have some income, without ctr there's little point, it's 'leaving money on the table' as the internet marketers say.
If you have an idea then try it, it doesn't cost much these days to buy and host a site but getting the right sort of visitors is the hard part, without them it's going to fail I'm afraid. Don't let me stop you though, you might succeed and prove it's possible. There's even some good domain names available so it's do-able. Live the dream.