They are a consignment business operated out of NYC. Perhaps a few of the tpfrs that have used them in the past can chime in? I have never purchased or sold through them, but I know there are a few tpfrs that have.
I bought a Bulgari bracelet from them a few years ago and Bulgari in Palm Beach confirmed that the item was genuine. So, I am confident Portero is legit.
I had a look at the site, and I think Jennab might be referring to their quoted retail price being dishonest? For some items I find it impossible to believe the quoted retail price could be that expensive, hence they're inflating the perceived discount you're getting from retail.