I normally post gold for coins on the site
and now I can't, the account has the premium rank
a message pops up : A minimum of 5 coins is required to sell via the trading system, you have donated for 1
suddenly something has changed and I have to buy some coins again ?
What part of the message "A minimum of 5 coins is required to sell via the trading system, you have donated for 1" are you having problems understanding?
Check if you have picked all options correctly, restart the browser, relog your account. If you have been selling so far, there is no reason you shouldn't be able to sell now.
Being able to access Premium features by donating a single coin was an overlooked bug, not something intended.
What surprises me is that Trade has been set to a minimum of 5 coins. I know making an account Premium had been corrected to require the minimum of 10, I'd have expected Trading to have the same threshold.