If you are going to look at Ortliebs, I wouldn't buy used versus new. If you utilize E-bay and Google shopping, and be patient over a little time, you can catch some good deals on them for pretty much what you will see them go for used on Ebay or Craigslist anyway. They don't lose much value if they are in good shape, but if you can get them at even "almost" the same cost as used, go new so you have warranties and know for sure there is no problems, leaks, tears, etc...
As for what the guy said above about the Arkel's +1 if you can find a set of newer styles used for a good price. They are another type that hold their value and hard to find for sale used. Or at least that was my experience.