Take the urge to purge and go with it if you're considering putting the house on the market. Not only should you go through all of your closets and donate the clothes that don't fit, are out of date, or just don't get worn anymore but you should do the same in your kitchen, garage, linen closets, and any other nook or cranny where you stash stuff.
You will also need to really scrub your space like your mother-in-law is coming for a visit. Consider hiring a professional cleaning service because it should be THAT squeaky clean and many of us don't have the time / ability / desire to roll our sleeves up that far. Recognize this guy? It's who you need.
When people walk through your house, they are going to be snooping so get ready for them. They are going to judge the quality of your house by the way you keep it. It might not make sense to you that the cat fur sticking to your walls (actual photo below) should indicate the upkeep of the big stuff like foundations & electrical systems but buying a house isn't always logical. So clean the cat too.