Exactly, but a lot of people don't understand the numbers.
If you have a 2% STR (and it might be lower the first few years) then those 2 out of 100 domains you sold have to pay for the other 98 unsold ones. So if your domains cost $100 on average, then you need to sell them for $5,000 each just to break even on the base cost. And that's just simple math and doesn't include any commissions, renewals or other fees.
When it comes to investing, only the revenue and expenses of your entire portfolio matter, and you can pretty well ignore individual sales.