Hey Luca, I think others have already pointed out the best way to get this idea working. But just a quick bit of advice, if you need it - if you're finding it difficult to find buyers for ice cream or any other food/item for that matter, which are not scripted, but can be roleplayed, maybe you could take the ice cream van and sell other products from it too. Perhaps other snacks, like donuts, drinks could be added into the bargain or just sold separately.
(Plus, those that have played the GTA VC single player may remember the "main" use for that van... :P)
That way you don't have to feel like you are asking money, essentially, for nothing.