The scam back in the day* was that they would bid on their retailer's own keywords. Their ads would send that traffic through their own affiliate links and get paid on any revenue generated. The ads looked legit and appeared to point to the retailers so most folks never even knew. So they cannibalized their retailer's own paid traffic and made it more expensive to bid on their own keywords.
*Back in the day was nearly ten years ago. Things may have changed.
I see other folks have already explained what I would call the "pop-under" approach where a new window is opened from RMN's site to the retailer in question. This is nefarious but in a slightly different way than the above.
Most affilite programs forbid this, but you can find that some clueless program runners allow it... usually because of site owners who are too cheap and clueless to bid on their own keywords anyway.
*Back in the day was nearly ten years ago. Things may have changed.
I see other folks have already explained what I would call the "pop-under" approach where a new window is opened from RMN's site to the retailer in question. This is nefarious but in a slightly different way than the above.