Which state is the property in, since each state has some slight differences in the property laws?
Has the seller actually withdrawn from the sale, or just expressed an interest to do so?
Have they given a reason for wanting to withdraw from the sale?
If the contract was subject to an existing sale, then I assume the buyer has a certain number of days to sell their own property, otherwise the contract will expire. How many days was this clause?
If there is a clause "subject to the buyer selling their own property", the seller MIGHT have also added a clause such that if they get another buyer, they have the right for the original contract to be no longer subject to the buyer selling their own property, and if the buyer doesn't agree to this, then seller has the right to sell to the new buyer.
Do you know if either the buyer or seller were placed under undue pressure to sign, or were there any aspects of the offer to buy and actual signing of the contract stage that may give cause for a fair claim to cancel the contract?
The standard contract usually provides for the entitlement of either the buyer or seller to seek redress or compensation if the other party defaults.
Do you believe your contact price is fair value, under valued or over valued compared to the market price?
Given that no deposit has been paid, if the contract price is at fair market value or over fair market value, and given that the seller doesn't want to sell to you, and there is still the need to sell the original property, it may be worth considering accepting what has happened, rather than spend money pursuing the enforcement of the contract.
Any buyer or seller can cancel or attempt to cancel a contract, the question is whether they can do it such that they avoid ending in a legal fight. The first thing to check would be the actual contract the buyer and seller have signed, and see what it says about either the buyer or seller defaulting