If you are thinking of selling physical goods from within an app then you should take a look at SurveyMonkey’s latest survey of over 600 mobile phone users.
The main conclusions are that mobile still suffers from end-user security concerns, Paypal is the most well-known payment solution and people re using mobile payments mainly for convenience.
I think that, as was the case with the web, people will eventually come to trust paying by mobile. The convenience thing is interesting because many solutions currently aren’t that easy to use. Companies that are well placed to introduce simplifications and improve on the user experience will probably be the ones that have the initial successes.