And you can resell their DRM free ebooks. Just let that sink in for a minute.

Do they make you promise not to resell a book more times than you bought it?

Unsurprisingly, yes. You have to delete the book after you sell it. Relevant quote: "Resale: If you buy an O'Reilly ebook, when you are done with it you may resell it, provided that you do not retain any copies of the book after you sell it. This is the same as the situation when you sell a used print copy—when you sell the copy, you deliver it to the buyer and no longer have a copy in your library. If you have bought a hard copy/ebook bundle, you may of course retain the hard copy, if you sell the ebook." - Under more about usage. Still an awesome policy.

Are they if I buy them through Amazon?

Buy them directly from O'Reilly. Tim O'Reilly and crew were early and full-throated in their support for DRM-free content, and they deserve your support.

You can also watch one of their pages/blogs (it used to have an RSS feed, too, IIRC) for frequent sales, if you are looking for discounted prices.

Dunno - I don't buy ebooks from Amazon. See above :)

More to the point - why would you?? Going via Amazon is just adding one extra step to a transaction. Buy direct.

They don't have DRM