You raise a good point, Alondra.
But I'm thinking that when Willie went shopping that if anyone DID see him and actually questioned it, they probably just figured that he was buying those items for his personal comsumption, and that Barnabas didn't eat it. Or that possibly Barnabas didn't really eat too much at home because he was always out of town on business. And we know that he liked takeout anyway. *wink*
But, Willie's diet seemed to get better over the course of the series. When we first see him he's skinny, or thin would probably be a better description, and he probably did each canned foods much of the time given the arrangements of the kitchen, like Alondra pointed out. But as we move further along, Willie starts to plump out. You have to realize--he stayed a Windcliffe where I'm sure he recieved sufficient portions and basically did nothing (well, not the strenuous type of work that he did under Barnabas' care), and later moved out of town to be on his own, where no doubtedly he ate numerous different things, not just canned goods.
In related news: Last night I was watching some episodes, and I think it was episode 1069 or 1070 where we see Trask in the PT Collinwood's kitchen sharpening a knife and talking with Amy, then later in the same episode we see Elizabeth and Roger (I think) talking in the room. And the kitchen is also used to roll the credits.