My advice is that you should buy the books online right now if they are available on the website. This is if you plan to save money, as opposed to buying at Textbook Plus or the UCLA textbook store.
Keep in mind that not all textbooks or manuals needed can be purchased online though.
Textbook vendors who sell online know when the peak periods are for selling books. They have the ability to set a quasi-monopoly in certain textbooks. The time when books are most expensive are the weeks when students start school (mid August - late September). I remember watching prices raise by 10-25% during these periods.
For example, a new book I just bought today is selling for $65-70, and could reach $90-100 by early September. The book costs $150 at the UCLA textbook store.
From my experience, purchasing engineering books overseas was the cheapest. The next cheapest option was from Abebooks, followed by Amazon, and then followed by Half.com.
Following the model I mentioned above, you now know when the best time to SELL your books.
I used to find the cheapest books on Half, but now sellers on Amazon tend to sell their books cheaper by a few dollars.