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 (daily reset) and lasts 24 hours. Note the auction duration may be extended slightly in case of bidding.
There are only five slots for auction of factory per day currently. Only the five highest level factories every day will be put up for auction among the ones that have been requested for auction.