"I'd guess they've had a couple of purchases"    -  More like they had a couple of red wines too many on a Sunday night!

Lucky them!!

I'm not too sure what an insidious seller is though.

A case of somebody liking a word but not understanding it?

That's what I wondered too.

My understanding of the word is it usually is applied to something more gradual, happening over time, such as a disease etc

If I had to say anything on ebay has been insidious I would probably say ebay policies have changed over the years, gradually getting tougher on sellers.