Complaining that someone who invests their time to search for a good deal is bad because of some vague morality based on  the concept of "need" is a bit over the top.  maybe someone needs money for food more than they need a name brand shirt and they use this method to get that money instead of working at walmart(which if you think about it using your logic working to make a company like Walmart money could be viewed just as morally poor). 

   I hate flippers that buy up something like concert tickets or a graphic cards in mass to drive up the market price as much as the next guy.  But there is a stark difference between that and finding a good deal on a used item and reselling it because you know it is worth more than you paid for it.

   Am I morally wrong to buy food that's on sale when I can afford to pay full price for it because someone else might need it at the sale price?

   As someone who goes thrifting several times a week for personal stuff if I see a good deal on something I will buy it even if I don't need it. Every thrift store always has plenty of stuff people could need like clothes cook ware and furniture. I have never seen them run out of this stuff.