I've been selling on here for a little while now. I'm not even close to $400 a month, but I can give you some advice.
1. Upload New Models Consistently / More Models = More Sales
- Quality should always come first, but you should be aiming to upload a decent amount of new products over the course of a month. Sales (especially organic sales from the site) are few and far between. The best way to increase sales is to increase the amount of items you have for sale. Again, quality is your chief concern, but if you can produce quality products, your next big concern is producing a lot of them.
2. Price High, Sell Low
- You want to make $400 a month. Here's a simple guide to get you there.
$400 = $1 product sold 400 times.
$400 = $4 product sold 100 times.
$400 = $40 product sold 10 times.
$400 = $100 product sold 4 times.
$400 = $400 product sold 1 time.
If you PRICE HIGH you can still reach your goal SELLing LOW quantities.
3. Failure is Feedback
- You may upload models that simply don't sell. Don't quit, until you've fully determined why you aren't selling. There are a couple reasons this could happen.
1. Quality of Model. You need to be realistic about the quality of your work. Look for forums and communities where you can get honest critical feedback about your work.
2. Quality of Product. Just because you have a good model doesn't mean you have a good product. Make sure you have good renders, descriptions, tags, etc.
3. High Competition. Some models get over done. If your model isn't selling, there may just be too many of the same model.
4. Low Demand. If people aren't looking for what you are selling, you're not likely to have a lot of sales.
If the problem is that the quality or your models simply isn't good enough, you may not want to waste your time uploading them. This can be a time consuming process, and your time may be better spent elsewhere.
All in all, if you have decent ability making 3D models, I definitely recommend that you sell them. With the right models and the right prices, you can reach your goal, but be realistic about what the process looks like.
Hope this helps!