I used these people last year in Italy (Tuscany) and was impressed.
http://www.cvtravel.c...EN/Pages/default.aspx
I also looked at James Villas and found not much between the two, indeed a number of the villas I looked at were in both brochures.
Having quizzed my Italian hosts as to how this all works (ever eager to get a better deal on another occasion), it seems the owners have to commit to a set of weeks they give over the trade (usually in the peak of the season) but the owners have some flexibility to do their own thing in the shoulder seasons. That is where I think owners direct comes into play - you can get a cheaper deal but only in some time-frames