It was a stolen unit. As the article says, best course of action is to go back to eBay and report it.

I only wonder if there is something a potential buyer can do beforehand to check if a unit is in good standing with valve.

This is the problem. These services have successfully pitted you against the customer. If you are not happy with the pay, your issue is with the employer, not the customer.

These services do pay drivers and it’s not enough. Instead of paying more, they redirect you to the customer for the rest, and in some cases you just get screwed. But it’s the service screwing you, not the customer.