I take the insurance option usually.
If I get two people interested at the same time (which has happene), it is first with an acceptable offer. I'll certainly let the other party know that it sold, or thank them for the interest. I've literally had this happen; (read first message) "I have some questions, want some pics, etc", (read second message) "I'll take it for asking price right now. Message me.". Sorry first guy, but sold is sold, albeit he's not going to know that's how it went down, but there's nothing unethical about accepting an offer versus interest. If you promise something and then go back on your word, well that's different. A good offer is a good offer, interest is only interest and thanks for it.
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