Leaving a website address within feedback, is this allowed?

[emptied.grannys.attic]

Hi, I started to bid on an item earlier this week, and checked the seller's feedback. I noticed something odd. He/she has a lot of feedback from the same person, and they are both leaving feedback for each other with the same website address in the feedback. Is that allowed? Or isn't that some type of free advertising? Also, when I saw 26+ feedbacks back and forth between the same two people, I immediately thought about shilling. No idea what is going on.