The unemployed person in my scenario is willing to forgo the opportunity to buy food, pay rent, or buy literally anything else. The tech worker is paying a negligible opportunity cost, as they have tens of thousands in their checking account and paying $600 has no effect on their ability to purchase food, rent, or anything else they want.

If I as the merchant believe the amount of money someone is willing to pay is equivalent to the opportunity cost they're willing to incur, I have to assume the person offering $600 wants it more.