You should reset the data of your addons once in a while. Why? Things get outdated, their prices change and after a while you'll look at a tooltip and see that obsidium ore's price is 90g per stack, when it's going for 40-50g/per at the auction house. New expansions, patches and other things affect the price of things and if you're running of scans are are 3 months old, or even go as far back to Cataclysm release, you're most likely running on data that's not accurate.
You'll want to reset your addons, and you can either do so by uninstalling and reinstalling the addon, that would be simplest and most effective way. If you're looking to reset one of the most common addons, auctioneer, use the following command in-game to reset it.
- /auc cleardata ALL
If you don't have any data, start scanning! It's a great way to obtain the price of items instead of running to the Ah and looking for said items. Additionally, if you notice that some items are priced below normal, buy them out and flip them! I learned a couple things from Markco myself, and one of those was to buy low and sell normal, and this strategy works! I couldn't of have done it if I didn't know the price of said item I was flipping.
I simply cannot stress how important it is to have accurate, up to date data in your addons. What are your thoughts on resetting your addon info once every 3-4 months?