I just missed a decent first edition set (ex library) at around $90, so it can be done: you just have to be quicker than I was! Alibris are probably ok if you're in the USA and buying from abroad, but their markup can be huge; being in the UK, I wouldn't touch them unless I was desperate. Many dealers on Alibris also list their stock on ABE or Bookfinder, and you can generally buy direct from them for considerably less.