Seems to me like this would bolster plausible deniability for all. I'd pay a little more this.
To enforce this policy, the site could provide the addresses directly to the vendor. Some could be actual recipients to verify the vendor is fulfilling their obligation. Some addresses could be targeted repeatedly to establish what appears like a real pattern of ordering illegal substances.
It would be hilarious if you were on parole for drug dealing and kept getting 1/4 kilos in the mail. Eventually, you'd be able to prove you're not responsible, and it would be a story for the news.
I would not use that market.