I guess it's a case that sometimes something isn't worth selling unless you can get a certain amount for it.
I have my original M seats sat in my back room of my flat. They look brand new. I'd be tempted to sell them but tbh, I also want to keep them as I like the option of returning the car to standard when the time comes to sell the car on (since the Recaro seats will likely not be to everyone's taste/requirements).
Therefore if I were to sell them they'd have to go for a price that would make it worthwhile for me, and also make up for the reduced saleability/value of my car later in life, otherwise it's not worth it.