Tom,
I think 50% would be your best expectation - as you know, it's a lot of work matching buyers with just the right book. The last thing a dealer wants is excess inventory, particularly in the current economic climate. Large purchases are even more difficult now due to same. My guideline on collection buying (and trust me - I'm a small fry) has always been: make sure you are able to recoup the total cost within 3-6 months. Then you can take your time and parse out the profits on the remaining inventory (if you have the space and a patient and loving wifee )
Also, my guess is that you have already sold many of the "easy" sells, which makes the collection on the whole less attractive for outright purchase, and more attractive for consignment. As to 30%, that is probably about right for consignment, as 50% to 100% profit margin is the goal for resellers.
And finally, NO I wasn't watching the USC/Cal game . . . but I did watch some of the cal/OSU game yesterday
bestest,
D