has anyone tried buying one of your own images to see if you get the credits or royalty?

I like the question!

Of course it is against the terms and conditions.

But I guess, the idea behind the question is only to prove, if sales reported always or not. Lets say: If the agency did not report about it - it would be also against the terms and conditions, right? And how could we prove it from contributors side if not by buying some images for proving it.
As long he only buys 3 images every three month, why not?

But I would not buy myself, I would ask friends to do it and indeed not many, because that could help ranking and so on and that is indeed against the terms and conditions too.

OK, I've logged in after god know how many years, my account shows 0$ balance although I've sold 14 images. I've never made payout there, so, where's the money? I should have 2$ on account. That is, in 6 years.

That's against the terms and conditions.

But that doesn't stop your friend from doing it for you.

It's stupid you have to buy your own artwork, agencies should have option, that you could download your images, the only I know of, that has this option is Depositphotos..