Get more than one Realtor to give you an idea of the market value. As to Walmart adding value, that would depend, will it back up to your house?? If so I would bet not!!
But if you do not have shopping close to your neighborhood, within 15 min, then it just might add value. This is something an experienced Realtor would be able to help you with.
Condition and upgrades add value as well and it doesn't have to cost you an arm and a leg to upgrade. Ask your Realtor to check on the most common upgrades in your neighborhood and add them if you can. Your neighborhood maybe going through a slump right now, like so many others, it may just be that you need to wait awhile longer to get what you need out of the house.
It's a slow market now and buyers are looking for deals. Ask another realtor or two to come see your house and to bring comps in the area to see how they compare to yours.