It's just my opinion but it seems like yes there are some who are earning their living selling models.
There are factors involved.
How much experience do you have?
Is your product in demand?
What price range is your product in?
Where do you live?
Experience is at the top of the list as you need to be able to upload high quality product as often as possible. Some products are in higher demand and bring higher prices, like game ready assets and good character models. Products in the 20 to 50 dollar range are necessary. Location. Easier to earn a living doing this in a place where the cost of living is low.
Having said all that, I'm an intermediate, my product isn't in high demand, all my stuff is in low price range and I live in Canada. I have been here over a year and made about 75 bucks so far. Not complaining though as it's all on me.