Frankel's camp claims she was in talks with eBay and an ad agency about running an ad featuring clips from last season's 'Real Housewives of New York' ... but no deal was ever struck. However, the commercial still went live on "multiple media outlets" last week ... according to the docs.

So, Frankel fired off a cease and desist letter to eBay demanding they pay up for using her name and likeness or yank the ad. And she says even if they pull the spot immediately, they'll still owe her for the week it aired.

As for the ad -- it's a spoof featuring fellow 'RHONY' star Dorinda Medley ... who's apparently totally cool with being in it.