From SFWiki

This BART station connects directly to Amtrak. Inside the BART station is a ticket machine for Amtrak that takes credit cards or debit cards (no cash). If you only have cash, then you can board the train without a ticket and pay the conductor. There is not extra fee for this since the Richmond Amtrak stop is unstaffed.

A few blocks beyond the Richmond BART Station are a few restaurants and corner stores where hungry travelers can buy food.