Luckily I'm yet to need to sell any transformers that I have a personal connection with.

I generally sell MISB spares so don't really mind where they go, I think if I had to sell transformers I didn't want to sell, I'd be hoping they went somewhere where they would be valued and enjoyed.

I don't use Gumtree, eBay is a necessary pain, here is a great way to sell, but compared to eBay a limited audience (who probably also just bought the same spare I'm trying to sell off somewhere else).