It's definitely the way to buy books - only problem is all the 'bulk' book lots I see at auction (and there are plenty about) usually don't contain anything I want. But if you're just starting a library it's ideal.
These were all priced at 50p or less if you had a bagfull. Best buy of the day an 1875 edition of the jubilee memorial of the Stockton & Darlington Railway. I got plenty of reference books for less than 50p including RCTS and OPC ones.