When sorting a pile of used pieces you will find some connected together. The good things is that they were almost all designed to come apart. Almost all. The one that I come across the most is the Bionicle Head and Brain Stems from the original Bionicle line. Unlike most LEGO pieces, once these were connected they are going to stay that way.
Even though these really are supposed to stay together, Bricklink has no listing for this assembly. Unless you sell it as a Custom item, there is no way to list it in your store. You will pull and twist on these to no avail. You can build the devices that others have built from LEGO pieces to pop them apart. They do work but my hands were sure tired. This is the easiest way I found. WARNING: DON’T USE ANYTHING WITH A SHARP BLADE!! DO ONLY UNDER ADULT SUPERVISION!!*
Find a butter knife or another non sharp object. It needs to have a tapered handle so there is a smooth transition between the blade and handle. Slide in the butter knife between the back of the head and the brain stalk where the axle goes in.
You can twist the blade and most of the time these will pop. Or you can slide the assembly down to the thicker part and these pieces will be separated back into two. I have had some that really stuck together so sliding the assembly down the butter knife was the only way to pop them apart.
Now you have two pieces that you can list on Bricklink.
*(Not responsible for injuries to yourself or others)