AnnaS notes a difference between USage/UKage of marketer/marketeer. I know the word from Black or Free Marketeer, definitely "ee" for me. But I'm not sure about UK use of marketer, so I looked it up. Many dictionaries ignore both words but Miriam-Webster Colegiate provides a subtle difference:
marˇketˇer Date: 1787
: one that deals in a market; specifically : one that promotes or sells a product or service

marˇkeˇteer Date: 1832
: a specialist in promoting or selling a product or service

and since the UK obviously only has specialists, no hoi polloi (or even fish tanks with koi polloi) for us, we use marketeer

Rod