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There is a discussion thread titled Multiple Listings, Same Author in the Book Bazzar- you can post there and indicate you want to keep the set together and maybe offer a deal, such as 4 books for 3 credits.
Another idea is to just post the first one in the series and then when it is accepted by someone, PM them and tell them you have the whole set available. Chances are they will want the rest of them as well and you can work out a deal. I just did that with a set of 3 I wanted to keep together. I was getting no bites on the WL Multiple thread or the Paranormal thread for over a month. Then when I posted just the first one in the series into the system, the person who accepted it was thrilled that I had the next 2 books as well and I did get 3 credits for the 3 books. Plus she took another one from me as well so I was very happy to save the postage.