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I received an offer, which I accepted - but nothing else has happened! Is it likely to still be in the queue, or the customer has not yet paid - or could the customer have taken back his/her offer?

The "Best Offer" system isn't perfect. Buyers can make an offer and then never come back to buy the article. But, unless it is in "waiting" status, your article is still available for other buyers. If it is in "waiting" status, the transaction is in process, which authors might see a little more often since there is a new procedure that creates an extra check for new buyers (to prevent fraud).