It of course depends on the item being sold and the perceived value. There is a lot of market psychology out here.
Wal Mart of course does operate on lower profit margins, lower than mom and pops because its wholesale prices are less to begin with. I have known businesses being put out of business on the wholesale ed because of Wal Mart reneging on contracts over pennies.
I shop at Wal Mart. I do not regard them as evil. But some of their business practices are cut throat to say the least.
To get back to the Mom and Pops. If Wal Mart can buy 200K units for its store, its going to get a price substantially lower than the local store that can only buy say 20 units. Hence from the get go the wholesale price is lower for Wal Mart and its pricing is reflected against that of the local store. Is that evil? No, but its very frustrating for someone that has products in competition with Wal Mart.
Sometimes I think of a small business plan such as on ebay, but the competition is so fierce it seems impossible to make a profit...so I fight the traffic and come to this job.
Even if one made something unique that will turn a profit the copycats would be on it like flies on roadkill.