Does anyone know what is this?

I think eventually, we will have to pay them for "having the chance" to have the photos on their platforms...
 

Wow. I would contact Alamy support asap.

FYI, Alamy replied saying that these photos have been used for internal testing only.

Alamy's always good about replying.

For as long as they've been in business, and given they allow downloads for customers without triggering a sale (that waits until the customer "reports" it as used), why would multiple, test, no-cost sales show up in a contributor account?

You have test accounts or systems for such things - something that doesn't interrupt the business while you work out new software or products. And doesn't waste support people's time answering questions about what this is.

This was probably new developer not aware that internal customer accounts for testing software features already exist; so he/she created new one & made couple of "purchases" which then got reported to outside world

Probably. The reply is also saying the technical team is apologizing.