Actually, I found brass that has been thoroughly cleaned allows me to inspect it much better than dirty brass. The dirt can hide pits, hairline cracks, and all sorts of penetrating corrosion. And, surface corrosion that will vanish with a light cleaning can make perfectly useable brass look pretty bad.
If you can clean it yourself, then you can buy dirty brass a little cheaper.
But, a lot of guys buying brass don't have good cleaning equipment. So, a seller can usually attract more buyers with cleaned brass.