I think I had somehow the impression that Karenin settled Stiva's debts after he married Anna, which is why I said "sold" into marriage, but I'm clearly mixing this up with something else!

I think Anna is after more than just "excitement" - because it seems that having an affair would provide excitement (and is not at all uncommon in her circle - the scandal is not that she has an affair, but that she makes it all public by running away with Vronsky (which I think is part of what Karenin has difficulty forgiving her for.) Princess Betsy has no problem with illicit affairs; it's Anna's need for love and her subsequent public "outing" (if you will) of the affair that is the problem for Betsy's circle. If Anna had been a hypocrite, she would have been all right!

Heh! Now I really want to read the novel again!