Using alt accounts to sell the same thing
I don't think this is against the rules.
Yes it is. You can't skirt the 'one thread' rule by having half a dozen accounts to sell the exact same thing. It's pure spam, not to mention using dozens more accounts to spam bump his multiple threads every day and also ban evade whenever he was banned on an account.
Is there a written rule somewhere that people cannot sell the same item via multiple accounts? Or is this something that is more along the lines of a mod's interpretation of the rules?
I agree that using alt accounts to bump your own threads (especially well in excess of once per 24 hours) should be frowned upon (I can see value to answering your own question provided that the question and answer happen very close to each-other -- and this does not happen frequently). Obviously ban evasion is not allowed, but that is a very different rule.