As per the Gamp's Law, a wizard can not create good food out of thin air BUT (s)he can multiply the food if it is available. We have seen many examples of such incidents where the food multiplication was used as per the plot convenience. For instance in year 2 when Harry and Ron were being reprimanded for flying the car and McGonagall told the duo to have sandwiches and pumpkin juice while they were in the Snape's Dungeons. The sandwich platter and juice jug kept on refilling till the boys were done.
Now one can say that McGongall simply charmed the platter and jug to fetch the sandwiches and juice from the kitchen and they were not created out of thin air. Though it was not mentioned in the book, a fair point and can be considered. But in year 6, when Aragog died, Harry kept on refilling the Wine bottles brought by Slughorn so that he should reach a vulnerable state and then can be tricked to share the Horcrux memory. In this case, Harry clearly wasn't fetching wine from anywhere. He was conjuring it out of thin air.
Now my question is why this practice is not abused? Why not buy couple of chocolate frogs and then keep on multiplying them at your wish? Why not buy one box of Bertie Bott's Every Flavor Beans and then keep on multiplying the beans at your wish? Why the Weasleys Wizard's products (puking pasties and nosebleed nuggets etc.) were not purchased just once and then multipled again and again? There are so many opportunities for wizards to multiply things, not just food, and then use them as per their will/leisure - why this is not done?
One more important point is that Why wizards don't use this trick to get filthy rich - in a legal way I am not talking about multiplying Galleons, although, this can be debated as other non living things are multiplied then why not galleons. But this has been discussed before although I didn't find the arguments that strong. But this is for later.
My idea is that let's take the situation of the Weasleys. We all know that weasleys are poor (I don't know why as they are wizards and can multiply things magically), nevertheless, a simple scam could be to meet a Muggle family having a wizard kid (Hermione is a good option). Now weasleys can multiply food and other elements magically, why not multiply a sack of wheat/rice or a bottle of wine or juice and then give the containers of these items to Muggle family who can sell these items in the muggle world, as regular seller, get money, pay taxes to the muggle govt and then share the rest of money among themselves?
In book 1, Hermione parents were exchanging the muggle money into authentic wizard money from Gringotts. Why not Weasleys (or any other muggle family involved in this LEGAL scam) use their share to get it changed into Galleons and then get filthy rich in a matter of days?