I have not heard of them, but that is the Venere map they have on their home page, so check to see if they are an agent for Venere listings (and compare the prices). Link to Venere
We have lots of listings for agencies for Rome and reviews of apartments on the site: www.slowtrav.com/italy/vr/
Rent Rome isn't reviewed but I think what Pauline was trying to say was that many times the same listing will be on several websites. And that the price can vary several hundred euro depending on what the agency charges for renting an apartment over the "real cost". So no, you won't find rent-rome on Venere but you will in all likelihood find the same apartment for a different price. Possibly considerably less.
My own experience (and believe me I've spent years shopping for the best price) is that vrbo.com is the best source for reasonable apartments. 99 times out of a 100 there isn't an agency taking a cut it's the owner who is doing the renting.