So I just had three books from two senders go missing within about four days. Never happened to me before. So PBS put my account on hold until I checked a box that I agreed to "the terms and conditions of membership" or something like that. Does this mean I'm on a kind of pre-bad swappers list?

No. It means that they're just checking to see that you're still active. Three books gone missing in less than a week probably triggered a red flag.

Not at all Nora, but I may be in your shoes soon.  I've had alot of books lately take a long time arriving.  One already went lost and arrived the next day, so I marked it from the archive (it was mailed at least two weeks late).  I've 2 or 3 more that are scheduled to go lost within the next week, with no enroute scans yet.  I realize that some books make the entire trip without enroute scans...but this many all at once has me a bit suspicious.

If someone loses his internet connection or ends up in the hospital, books they've ordered won't be marked received, and there will be no response to requests and wish list offers.  We don't know what the specific events are that trigger someone's account being placed on hold, but it's not a punishment.  It's more to protect other members (from having books they've mailed not marked received) and also the member in question (from having books removed from his bookshelf and wish list when he doesn't respond.)

One thing that helps is if you're communicating with the sender.  You can send a PM that says "Wanted you to know that the book hasn't arrived yet, but that I'm watching for it, and will mark it received when it gets here.  Please let me know if it was returned to you, or if you have any other info about it."

I have contacted both senders. No reply from either one.

That's good -- if the Admin Team looks at the transactions, they'll see your PM, and the non-response from the senders.