Who's paying for it? Are they working for free?

The realtor might pay for it or even do it themselves. It would take 5 minutes with a reciprocating saw. Or the scammer tells the realtor "never mind that" and the realtor tells the buyer.

No, but paying someone $300 is cheap when you hope to get a check for several hundred thousand in a few months. (even if the scam is only to get the earnest money that is still a $300 investment for the final thousand or two you make - with very little work)