Well, the markup/markdown is a bit excessive in Skyrim, but anybody does the same thing in the real world. If you sell something to a retailer of used goods, they're gonna give you less than what they're gonna resell it for, or they wouldn't be in business.
My question for the whole loop is always like this:
If merchants always buy low and sell high,
Can I be a merchant as well?
Because, as it currently stands, there is no way to RP as a merchant in Skyrim (and in Oblivion too). You can't go any farther than being a mere supplier
I find it a bit confusing :/