It took a few days, a few times back and forth to peakmonsters, to figure out the layout of the site. Today i started renting a few cards i have.
Clicking on your checkbox next to your card brings it into the menu on top, and shows the number of vards selected.
From there you can choose the price, i picked just the market price. You can also stop renting from same menu as well.
Figure i would try it out and see.