Who the customers are and what the want is the big Q we all want to know the answer for. CGTrader comes with some statics about what categories selling best and like that. That can be usefull Information. You can take a peek at the other sellers and see which of theirs models that is top selling. You can experiment with different styles and see what work. Most important have patience.
Some times I get a idea for my 'next big hit' and then it never sells. Sometimes I suddenly sell a model I have lost faith in.
For mine own customers, as far as I can gain Information, they seems to range from hobbyist, freelance designers to small production companies.