CPU usage can be a bit misleading when it comes to diagnosing bottle necks in your setup. For instance, GarageSale might be blocked by your hard drive delivering some requested data. In that case, CPU usage would be very low, but still your system feels sluggish.
If that’s the case upgrading your system to an SSD usually solves these performance problems. You can find out if your computer already comes with an SSD by choosing “Apple” menu > “About this Mac” > Storage. If the windows says “Flash storage” in the lower left corner, you are already using an SSD, otherwise you might want to consider upgrading to one.
Have you checked how many auctions you have in GarageSale? Maybe you can delete some of these, too. And don’t forget to use the “Empty Trash” command in the “GarageSale” menu.