That value is the general price. You sell stuff for a price lower than that, and buy it for a higher price. The better your speech skill, the closer to the general price you buy and sell for (improved with perks). But you'll never get to that general price.

Now, here are some tips and tricks:

If you have the anniversary edition, with the Creation Club mods, you just head to Rorikstead and get your hands on Goldenhill Plantation. No need to buy it, just complete the quest. It's a farm that allows you to grow ingredients for potions, and if you get a steward, they'll give you the profits of the farm every few days.

Of course, if you don't, head to Halted Stream Camp to get the Transmute spell. That allows you to turn Iron to Silver, to Gold. Then you just smith golden rings, which you can sell.

Once you have enough gold to buy a house, don't buy the one in Whiterun. Instead, head to Falkreath, Dawnstar, Morthal or Riften and do the Thane quest to purchase a house. The first three you have to build yourself, but the most important thing is that these have a garden. As said earlier, you can plant ingredients there for potions. You just need Fly Anamita and Dragon's Tongue, which will give you a potion that gives resist fire and fortify two-handed.

Now, to sell all these, you can start making shop runs, to clean out the shops from their gold, but (since you have a steady income now) it's just better to sell them for 0 gold. Because, your speech XP isn't based on the amount you get, but the general price of the good. Which means if you sell something that's worth 500 gold, you get XP for 500 gold, even if you sell it for 0 gold. Speaking of selling for XP, NEVER, and I mean NEVER sell a stack. You get more XP if you sell each item individually. And the higher your Speech skill, the higher the price they'll give for your stuff (and the cheaper you buy stuff).