People like to pay for convenience, no? If you consider all the planet trades on the forum and FB groups, it is a huge untapped market. I suggest to open up planet auctions much like the GTC is public, but bid using GP and take the 20% or whatever tax on the GP similar to what is done with the GTC credits. Seller sets starting bid (might as well start at reserve price); unsold planet costs nothing. I can almost assure that this would be a large GP-sink for the game but people are pleased to have planet availability anyway.

Alternatively, if the thought of some GP transfer that would occur is repulsive to your game design, simply make listing a flat fee of GP and the purchase price/bid in credits as usual.

Pretty sure all the loyalty measures are in place because Dan isn't a big fan of planet trading. but what do I know?

Interesting concept, unlikely to happen I imagine

Yeah I never understood that. I think the sentiment is irrelevant when examining it through a GP sales point of view. It is a very active part of the game and I feel where there is activity there is room for formalization of trading.