It's worth noting that $300 for a day of manual work is considered a good salary in many parts of the country (and even in San Francisco).
Once you factor in the girlfriend subsidy, they would have made better money with far less risk by getting shifts at local fast food restaurants.
Even if you take out $75 for the GF subsidy (being generous and assuming she gets an equal share), each guy made $25/hour, cash (i.e. ~$30-$35/hour pre-tax), which is >3x what they'd get at a fast-food joint.
Now of course, one does need to take into account the fact that they're restricted in spending their loot on beer, which they are obligated to share. From a short-term economic standpoint this is probably a significant hit on their profits (i.e. unlikely that the three gentlemen will consume the majority of the beer). However, from a longer-term/more holistic view, purchasing beer for the group will surely lead to fun, team building, general goodwill, and so on, which is arguably more valuable than the $300 cash at this point in the startup's lifecycle.
tl;dr startups should use aeron-chair arbitrage to fund investment in beer, it's science.
As a general rule of thumb, always ask for cash. Or ask for a cashier's check if the buyer is not comfortable with carrying large of cash around. Don't accept anything else (e.g. personal check or even paypal).
In addition, there are enough scammers lurking around on craigslist that I didn't want to expose myself to those kinds of risks.
I was also buying/reselling items with fairly low-margins. Credit card processing companies e.g. square, paypal, etc. will only eat away at whatever little profit I was making at the end of the day.