Am I flipping incorrectly?

Hey guys, I'll make this short. I have a 30mill stack, and I've been trying to do quick, small flips with it. Here's my process.

-- Pick an item (raw fish, cooked fish, ore, bars, dbones, dhide, dragon weps, whips, arrows, clean herbs, grimy herbs, potions, potion ingredients, summoning pouch ingredients, the list goes on.)

-- Buy one of that item way over price, the amount I ended up paying is my sell price.

-- Sell that item way under price, the amount I get back is my buy price.

-- Buy a few hundred/thousand, at the buy price.

-- Sell it back, for the sell price.

Now, what usually ends up happening is it takes a while (upwards of an hour) to get the item, and then I sell. Occasionally, I get it quickly, but then it takes forever to sell.

I've watched guides on youtube, and see people do 5-10 minute flips.

What am I doing wrong?