Selling option in the shop
The only way to earn gold is to log in every day, invite friends, or purchase it with real money, so I think there should be a shop option where you can sell your blocks that you don't want/need anymore for gold.
Superman(Because why not?) commented
You also get gold from killing mobs. This is entirely unneeded.
Farm gold by killing mobs at /tele zoo or /sellallinv