Most of the books I've purchased have been used, and purchased from eBay. I seem to have the habit of getting very interested in out of print games :)
Wait for a reprint? Will that happen?
Once a system has been out of print for a while, it becomes practically unheard of for a reprint to happen. The odds are good that if the property has any remaining appeal to publishers that a new edition will show up, instead of a reprint of the old one.
This is particularly true of properties that have been licensed from a third party (like Star Wars). Odds are that the license has lapsed, transferred to a new publisher, or is otherwise making things difficult.
Is this the normal, a few years after print, for the availability of books to dry up?
Generally speaking, no. Prices for both the WEG edition of Star Wars and 7th Sea remained pretty stable for the entirety of the time that I collected them (a period of around five years).
The core books will generally always be cheap, with hard to find "chase" books slowly creeping up in value.
The main issue is finding the books: At the moment I'm hunting for the Vendel splat book for 7th Sea, but can't find it for a decent price.
Most books will be in good condition. They'll appear worn... There'll be thumb creases in paperback covers, scuff marks, and perhaps the occasional scratch.
I have yet to receive a book that was seriously mangled or stained, however. I have no issues passing the books around the table, although I probably wouldn't put most of them out for display on my coffee table.
Of course, you're dealing with individuals. I've received copies that looked like they came right off a bookstore shelf. I would imagine a few people have received lemons. But by and large books are pretty durable and don't degrade much over a time-period of decades.
Should I buy a second hand core rule books?
Sure, if the price is right. If shipping costs are prohibitive, you may want to look into buying a PDF instead (although I'm not certain that Saga Edition is currently available in that form).
Off topic aside: If you can't get Saga Edition, you can always try adapting D6 Space; a refinement of the engine used in the original Star Wars RPG.