The words cellar, seller sound the same but have different meanings and spellings. Why do cellar, seller sound the same even though they are completely different words?
The answer is simple: cellar, seller are homophones of the English language.
A wine cellar.
An underground shelter, as from storms.
A room or enclosed space used for storage, usually beneath the ground or under a building.
A basement.
One that sells; a vendor.
An item that sells in a certain way: This washing machine has been an excellent seller.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").