In another post I showed a Penny Black that I bought recently. It's replaced the one I had in my GB collection, so I'm selling my old one shown in the pic
I chose not to list it on Ebay, where there's hundreds of Penny Blacks for sale and the competition is fierce, and listed it on a stamp forum instead. With a day to go on the auction, I've already recouped the amount I paid for the better stamp. I get a sale in three days, no merchant fees and money in the bank.
Staying loyal to one platform is fine, but think horses for courses, regardless of what you're selling