I think there are three basic things you need to focus on and balance to get a return on investment:
1. Quality of your names
2. Size of portfolio
3. Proper pricing
For a decent quality portfolio, the average sell through rate each year is 1-2% you need to make sure the math works to make you profitable in that range.
As an example, if you had 100 names you might sell 1 or 2 this year. Your renewals, on average, might be $1000. Therefore, your average selling price needs to be at least $1200 ( don’t forget commissions) just to break even, but better if your average is $2000 then you have a profit.
Frankly, the biggest issue most have is quality. Sure, there is some luck involved, but you can create some luck by having quality at an appropriate price.
Adding more domains to your portfolio beyond a hundred or two should only be done once you have established that your quality picking and pricing skills are profitable and consistent.