Some of it is just plain greed. I needed a 5 gallon gas can. Went to oreillys. $23. For a plastic can. Rural King had the exact same can. $12.
I needed a rear axle seal for an 8 3/4. Was about $3.50 from Rock Auto. Napa had it for $20. Got on the oreillys website. They showed a certain part number, and they had 2. Because I didn't bring the old seal, I get home to find they were for a Dana. They didn't list the part number I needed on the website. But, they did list 5 seals that the website said would not fit my car. The only way I was able to resolve this was someone on this site posted something about seals, and included the part number. Then I was able to go back to them ,and order the right part.
