We don't know whether these people are paying for each image, but at $5 for a "custom" logo, I doubt they can afford to purchase the rights for each image. Yes, if all of their logos were clouds it would work, but that's not very likely.
If we're to accuse someone of plagiarism, we have to present the evidence. Onus is on the accuser. OP has done a disservice to his cause by not providing any evidence.
Automatically assuming plagiarism due to a low price-point is pretty dumb. This is a legitimate website, and plagiarism would be a massive risk for business. If they sell 100 logos a day -- that more than pays for the extended licenses. (They could even be getting bulk deals, going directly to the original creator, special licensing agreements to move more stock, etc).