You can buy Premium account or gold on auctions. Auctions will be shown as you reach level 60. You can bet and win 100 G in the beginning of every hour and at XX:30 (in other words, every 30 minutes). You can also win Premium account: auctions for Premium start at 17:00/17:30 GMT and 5:00/5:30 GMT.
You can also sell your Factory on the auction: you can send a request from factory settings page. Only factories with level 100 and higher can take part in an auction. Auction starts at 18:00 GMT and lasts 24 hours.