I would strongly recommend that you do not do this. It is against Aeroplan's terms and conditions and therefore extremely risky. There is no way you can prevent the person whose miles you are buying from changing/cancelling your trip to shake you down for more money and there is always the risk that Aeroplan will find out he is selling miles and cancel the tickets, potentially leaving you stranded abroad.

If he is selling miles to you, he is probably selling to others and there are automated algorithms to catch this type of behaviour. It's too risky.