On this one I fully DL's position (and UA's of course). Before I joined flyertalk I knew this rule (as well as the fact that miles itself are non-transferrable). If you don't know it don't blame the airline. It's clearly stated and before you start selling your miles shouldn't at least check the rules? We always seem to check the rules to discover if one award is cheaper than the other, etc. I think the e-mail of Delta's RPU is still very friendly compared what they could do.
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Cheers,
Brenno.