What about a "kick out clause" to protect the seller?
It could allow the seller to continue to market the home to other buyers.
I can't imagine many sellers being very excited about having to wait for the buyer's house to sell-- if it sells. The buyer's current house could be overpriced, in a bad location, outdated, bad floor plan, in need of many repairs, etc. Or, the buyer's "buyers" might not qualify, might walk away from the deal, etc.
Personally, I wouldn't want to take that gamble on "indefinitely" waiting for a buyer's house to sell... and not being able to continue to market to other buyer's. Make sure the seller (and buyer) is protected. The average house sale time in OKC now is what, 6 months, for the market average priced house? If it's a million dollar house, less than half a dozen of $1 million+ houses that aren't new construction have sold in Edmond in the past 1.5 years... if I recall correctly. You don't want to be a seller waiting 1.5 years for someone's house to sell before you get yours officially sold in that bracket. A $150,000 house sits on the market much less long, on average.
If a seller is even considering a contingency offer, the contract should be carefully drafted to allow the seller a good "out" so they're not waiting "indefinitely" for the buyer to sell their house.
A good number of people offer based on contingency. You just have to make sure the seller is protected and not stuck.
A strong priced offer with a reasonable contingency duration might be a win-win.