Best practice is wait until you have a contract on your existing home prior to making an offer on another one, be sure the contract on the one you are buying is subject to being approved for financing. Make sure your financing is subject to selling your existing home.
HOWEVER contact mortgage broker determine if you qualify purchase another home IF your current home has not sold.
Lynn911 Dallas Realtor & Consultant, Credit Repair Advisor
The Michael Group - Dallas Business Journal Top Ranked Realtors
Otherwise it just creates too much stress for most people.
Sure things can work perfectly and sometimes they do, but when they don't it can be a receipt for disaster.
What happens when you have a contract for a new home and lots of money tied up with that in earnest, option, appraisal, inspection and the buyer for your current house doesn't come through for whatever reason.
That is unless you can afford two houses at the same time....then buy first, move, and sell the original vacant.
You close you sale in the morning and your purchase in the afternoon... busy day but a very rewarding one that ensures you are never homeless. Buying and selling is much more involved than just being a first time buyer... hire a Great Realtor to assist.