You need to balance your own costs with the market value. Claculate your own freelance rate and costs for making a model and establish an ideal Return on Investment date.
Selling several copies of the same model is generally a slow burn. Sometimes you can start seeing profits after a couple of months of having posted.
Do some market research on prices of similar items and compare to your own costs. If your quality is high, you can expect to charge above average. If it isn't, then you need to compete with price. Also, try to find a target market. Are you going to sell to indie developers or to huge studios?
As for content, do what you like, but stay within a realm of generic stuff if you'd like to sell more. For example, you may do an amazing aztec warrior, but maybe it won't sell as well as a generic knight as more people create European medieval style games. Even if there are already many models out, some companies are looking for more variation. On the other hand, if you do something original you may hit the jackpot and sell a lot.
Think it's about checking your investment and calculate a risk. Yet, can't harm to upload something you made for practice on your free time; perhaps it will pay off some bills.
