Apparently, if you get the old version of the Sims 3 (before 2012) on disc you can play without origin even if the game has been used before. But what about the expansion packs?
I just bought the basegame, pets and late night used and now I'm unsure if that was such a good idea. I don't want to register it, I'm just wondering if I'll be able to start the game without origin?

Yeah, you could - the key codes aren't tied to anything and are, in fact, so generic that you could pass the discs on to all your friends and they'd be able to install it just the same. As far as I know none of the patches up to and including 1.67.2 are tied into Origin, so you'll be able to install every pack without having to register or activate anything. Should you want to register your packs on TheSims3.com, though, then you might find that the previous owner has already used your codes and that you won't be able to get certain extra content.

yep, I second that, It's how I ended with "complete pack".
My wife hunted down 2nd hand packs in secret (it took almost a year), for my birthday. She knew I would not ever put so much money into a game (I've had only BG for a long time), so she has made me a surprise.
Imagine my face...

Libraries have even have had copies of some Sims games and EPs and everyone could install them with the code. As said, people just did not register them. Many of the "freebie" promotions that came with some things have long expired. I could not find my pets register for example and could not get again from EA. I then later found it which was good news.

I have the entire set of 2 on disks other than Ikea and HM and Holiday which I did not get. Have debated about popping them on ebay as I will never use again and others may want them? And someone else should have no problem using them?

So yes, used stuff works.

Hmmmm, I have the whole set of 1 too. Somehow it is hard to part with them though. LOL Too much history and good memories associated with the prior versions.

I keep needing to get Supernatural and Town Life on disk so I can get rid of the 1.69 patch

Yes, you can, but any free content is expired and there is a likelihood that the key codes were registered with the original owner.

Okay, everything is working as it should, I'm very happy. Just to be clear - I can download patches manually, as long as it's not 1.67 or later, right?

You can use any patch level as long as it's valid copy of the game.
I'd recommend using superpatch (1.67) if you can (no element of the game is tied to Origin) and stay away of this malware.