Maybe I've been reading too much Harry Potter fanfiction lately, but I don't understand why the Weasley family is poor. I mean, they can all do magic. They should be able to make a fortune with minimal effort!
Remember in Order of the Phoenix, where Dumbledore wants to sit down, so he conjures two fancy armchairs out of thin air? He literally makes furniture out of nothing! Am I the only one who thinks that Dumbles can use this power to become a furniture salesman?
Think about it! He can make any type of furniture he wants, within a couple of seconds, without any production costs! In one afternoon, he could make enough furniture to supply a hundred stores. Less talented wizards could easily get similar results, with transfiguration or the duplication spell.
Wow, the duplication spell. You could use that to give you an infinite amount of whatever product you wish to sell, at little to no cost. You could easily sell a huge amount of product to Muggles, then have that money transferred into Wizard currency. Personally, I would open a mail-order business that sells statues and religious artwork to churches at low prices.
An even simpler business would be a repair shop. My parents complain that it costs way too much money to repair and replace old eyeglasses. In the magic world, it took about three seconds for Harry's glasses to be repaired, permanently. How easy would it be for a wizard to run a repair shop? Just ten minutes of work a week should be enough to support a wizarding family.
In conclusion, the Weasleys should not be poor. They have plenty of no cost, little effort options which would allow them to make lots of money. Even a simple laundry service would be able to ease their pocketbook pains!