I'm pretty sure every registrar will look at the traffic/profitability of any domain that is in redemption. If the numbers work and the registrant does not renew the domain they'll pick it up and park it and make money off the traffic. As for selling it back to you, you'll now have to pay what they think it is worth which will vary based on how much money it is making them.
Ethical? Fair? Maybe not, but you did have a chance to renew it. Legal? I'd bet it is.
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