@ JackBaldy: Thank you so much for this information. It appears more clear to me. Anyway I'll continue to check the merchants to see what kind of enchanted items they have now. It seems that they have far more now than when I started that game. Too bad because I don't need them anymore.

It's nice doing this long checking work for us.... We appreciate.

So today, on a new Dunmer toon.... I saw (but couldn't afford) BOTH the poison and disease immunity necklaces - one was at Belethor's in Whiterun, the other was at Bits and Pieces in Solitude. Random is random....

It's possible that they fixed it with the 1.4 patch, you're playing a modded game which includes a fix for it, or you sold him a either of those necklaces so both of them appeared at the same time.

No, I didn't sell either of them as I'd never had either of them. Toon was about level 10 at the time.... Yes, I'm playing a modded game but none of the mods I use have anything to do with those items (body/hair mods; house mods) yes, my install is fully patched but I don't believe the patch notes indicated anything regarding those items.

The Necklace of Poison Immunity doesn't start appearing until level 15 and the Necklace of Disease Immunity doesn't start appearing until level 20. In any case, patch notes are generally not known to be 100 percent accurate. It's rare to find patch notes (In any game) that list every single change that has occurred.

Anyhow, things in your story sound a bit off but I can't be bothered to go ahead and run another series of tests for patch 1.4. Take the information I give you as a precautionary warning if you must or just ignore it completely.

Sources:
http://www.uesp.net/wiki/Skyrim:Generic_Magic_Apparel#Resist_Poison
http://www.uesp.net/wiki/Skyrim:Generic_Magic_Apparel#Resist_Disease

Well, you know.... some of the things that "only appear at level whatever" occasionally show up for some of my girls a lot earlier. Like daedric daggers. The Dunmer toon is the only one that ever saw the immunity necklaces at all.... and she hasn't seen any daedric daggers. *shrug*

You may certainly choose to disbelieve me. I don't care. It's as hard to prove something as to disprove it, and I'm not worried about proving it to you or anyone else. I just find this game entirely random, and therefore don't actually believe "set in stone" statements....

http://www.uesp.net/wiki/Skyrim:Enchanting_Effects