I cannot tell you how many times I have taken a client in to a house only to be told that it 'smells.' This can be a good smell or a bad smell, but anything that offends the senses will be noted.
The obvious offendors are cooking odors. Some that come to mind are fish, anything that has burned, fried foods, curry, and even cookies and cakes (suspicious buyers think you are hiding something with a good smell). This is a great time to eat out or grill your food outside...anything that keeps the food odor out of the kitchen and the air circulation system.
Mildew is another odor that will cause a client to turn around and walk out of a house. Keep your house free from excess dampness and deal with issues that would cause the dampness in the first place. Once a client can smell it, you are either losing a sale or losing money.
Pets, and their smells, can be a problem, too. At the risk of stating the obvious, keep the litter boxes and cages clean so that there is no smell coming from them. Damp dog can permeate the house, as well. Keeping your dog bathed and off the carpet when wet can help during the listing period.
A surprise problem that many clients don't think about is the plug in air freshener. A good smell to you may set me off on a sneezing attack! The aroma of this type of air freshener is too strong for many people, which can cause them to sneeze. Believe me, you are not inclined to buy a house if you are sneezing the whole time you are looking around. Further, these set of alarm bells for many people; if you have to use an air freshener, then your air must not be 'fresh.' You would be better off dealing with the problem (if there is one) and skipping the plug in air freshener. Even if you are just using one because you enjoy the smell, remember that you are trying to market your property...you might also like to throw your clothes on the floor, but at least while your house is on the market, you probably won't!
This is an area that your real estate agent can help you with. Ask for honest feedback about how your house smells--and take it in the spirit with which it was intended. Sometimes we become unable to smell things that we are around every day. A rule of thumb to keep in mind is that nothing in your house should detract from the house itself. This is true of the things that you can see, but the things that you can't see are often overlooked. You wouldn't want such a simple thing to be the reason that you miss out on a sale, would you?
Smells can be good or bad. Usually when a client asks me what a smell is I generally assume they think it is a bad smell.