Yes, many people do that, and I believe successfully. This may be a good business if you have at least minimal skills in editing and customizing themes you are going to offer to your clients.
Unfortunately some of such "businessman" are just lazy people who don't want to do anything useful, they just mediators. Sometimes when it comes to even smallest and simplest customizations (sure they don't have enough skills even for that) they just start spamming the authors with infinity requests, and if you don't meet each their wish they will start complaining (your theme is bad, my client is angry etc...) and give an item poor rating.
Of course, sometimes this is an authors fault, because not every item on TF of a good quality, so be careful here too. Read item comments section and check overall author's rating before you buy.