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 is a limited number of slots for factory auction. Only the highest level factories will be put up for auction each day among the ones that have been requested for auction.
Any gold withdrawing operation will now look for your winning (at the moment) factory auction bids.
For example, if you bid 200000 G and it is currently the winning bid and your balance is 210000 G you will be able to spend only 10000 G, not 210000 G.
Also, one player can have only one winning bid. You will not be able to bid in factory auction A if you currently have a winning bid in auction B.