Depending on what part of the country you come from, you may pronounce “sell” and “sale” the same way. So, incorrectly, this may carry over to your writing where you might use these terms interchangeably. While these terms are related to the same idea, one is a noun and one is a verb, so they cannot be used in the same way.
Use “sell” as a verb. This term should denote an action--trading some sort of property for some type of gain, usually money.
Use “sell” in these instances: I want to sell my old computer. That store sells kayaks.
Use “sale” as a noun. This term refers to a forum for the act of selling or simply the act itself.
Use “sale” in these instances: We made $40 at our garage sale. There were many Labor Day sales at the mall last weekend.