Stick to knowing only a few items
You only want to know the prices of about 50 items or less, in terms of having it in memory. You can always search the shop wizard or the auction genie to get a better estimate of items you aren't familiar with. However 50 should be the most you work with on a regular basis.
If you try to remember the prices of too many items, chances are you'll remember the old prices. Prices from week to week, or even day to day change. You don't want to selling items in your shop based on what the prices were a week ago.
Also, you don't want to be buying items, or bidding on them based on what you saw the prices were 3 days or a week ago. It's not uncommon for an item at 11k to drop to 9k in two days. It can also jump from 15k to 18k.
You don't want to be selling in you shop at 15k and since it sells, you keep putting it for 15k, even though you could get 18k. Likewise you don't want to keep bidding 16k or 17k at auction if you can't sell the item, and do a search on the shop wizard to see that "oh crap, the price dropped a lot".
I wish I could know the price of everything at all times. However, I can't. Plus the more things you try to remember, the more often you have to search the shop wiz or auction genie to "recalibrate" and check what the actual prices are. To me, it's not worth it. You waste too much time that way.