sell, cell, cel

Sell or sell means to trade for cash or money; to vend--
He decided to sell his old things at a flea market.
She learned to sell real estate and got a new job.

Cell or cell ( rhymes with sell ) means one of the units of a living organism--
Under the microscope, they could see each individual cell.
The hospital did some tests, but they didn't find one cancer cell.
Cell also means one of the locked rooms in a jail or monastery--
He had to share his cell with another convict.

Cel or cel means a transparent image used to make one of the frames of an animated film--
He bought a framed cel from a Disney movie to hang on the wall.
This old cel may be worth some money to a collector.

Now that you know that, you can say--
"He placed an ad to sell an old cel of the dogs in a jail cell from Lady and the Tramp.

Note--the past tense ( yesterday, or some time ago ) form of "sell" is "sold"--
He sold his old things at a flea market last week.
Note # 2--an organism made up of only one cell,  biologically, is called a "single-celled organism"--
We saw a single-celled organism under the microscope.