You can't ask somebody if something is morally right or wrong. So you must consult something called Side Constraints.
You can't do something if it hurts something else in the process.
I'm pretty sure that's worded incorrectly, but you get the idea. I can't create a cure for AIDS if I have to kill a human in the process. In the Neo world, I can't earn enough money for a Darigan Paint Brush if I have to scam people out of their NP.
However, what you're doing is artificial inflation. You're buying out other people's cheaper items so you can change the current value of the item, changing it from 800 to 925. Artificial inflation is against the rules.
But then again, 125 NP difference is nothing. It doesn't really hurt anyone; people don't care whether they have to pay 100 NP more for an item or not. Plus, you're buying other people's items, so they are still gaining NP because you're paying for them.
So is what you're doing morally wrong? You won't be frozen for doing something so small scale, but on a technical level, it's wrong. Morally? I think not.
But if I consult myself in this case, I do buy items at low prices then resell them at higher prices. It's called restocking
EDIT: I just reread my post and I realize how hard that would be to understand if the reader, well, wasn't the author. Apologies.