Make sure that the UPC on the item in store exactly matches the UPC that is shown on BrickSeek. Oftentimes, a new model of an item can look identical to the previous (and marked down) version. If the UPC is off by even one digit, the inventory counts and pricing could be different than what you saw on BrickSeek.
For example, look at these two 32GB memory cards. The packaging on the front and back is nearly identical; however, their UPCs are different. One of these memory cards was $5 on clearance, and the other was still full price at over $18. One is the "older" model and the other is the "newer" model. This is very common and this is why it's very important to always verify that the UPC of the item you have found is the same UPC as the one listed on BrickSeek.