This is an eloquent analogy about selling on eBay (and building eBay services):
If you choose to invest time building sand castles, you have to anticipate the waves. I’m not telling you not to build castles. Just don’t cry when they wash away. Get yourself a castle mold so that you can easily rebuild. Be prepared to plop another one down the moment the wave receeds.
The analogy is from seller exdwh: