I take the following approach to buying and then unloading collections - keep the keys or mark them up, put the commons at 50% of guide, then blow out the rest of the books worth even up to $10 in the guide out at $1 apiece. As long as you buy properly you will make your money back from the $1 books, double it with the commons and lesser keys, then keep the best of the books for a rainy day/future gains.
There are plenty of late BA into CA collections out there right now that can be bought for $0.50 or less per book that can be flipped readily for a nice profit. For example, last week I picked up a small 1200 book collection for $500 that is 90% late BA and CA. There is a nice run of New Teen Titans #1-10, DC Comics Presents #26 & #27, Star Wars #35 - 50, plus a bunch of other CA keys/hot books.