Hello @Larry N.
Yes, the Seller can dictate their terms and can refuse to close when they realize the deal is being assigned; BUT Subject To whatever the contract says. If they refuse because they are angry about just hearing about the assignment how likely is it that you are going to force them, or sue them, to compel them to close? Most people say that a contract is assignable, UNLESS it specifically precludes assignment. I have never had an attorney give an opinion on that. However, if the Seller tells title they are not going to close the title company is not going to hold their feet to the fire and argue contracts are assignable unless assignment is prohibited. Title is rarely going to get involved when a dispute arises. So, that leaves it to you to either convince or compel the Seller to close.
Most wholesalers I have experienced do not disclose that they must find a buyer in order for them to purchase from the Seller. I personally think it is not good business to tie up the seller's property unless you have a strong chance of closing on it. HOWEVER, I am a transactional funder so have seen many deals and can say that the most experienced and successful wholesalers I have dealt with are straight forward with the Seller. Obviously they must have a good pitch to the Seller such as "I do this full time - it is how I make my living", "I have a substantial list of investors who can pay cash", "this structure avoids realtor fees and the many showings you will have to accommodate if you list your property on the MLS", "this allows you to sell without making repairs".
Most experienced wholesalers put in the additional provisions section of the contract "Buyer intends to resale this property for a profit". Will these types of disclosures lose you some deals? - likely yes. Are these disclosures more likely to protect your backside and avoid sellers bailing when they realize the deal is being assigned? - absolutely. The resale for a profit language should be in all Short Sale transactions. More and more states are also taking a position that wholesaling (via assignment) is brokering real estate without a license. Check with a competent real estate attorney in your state. If it is an issue in your state you can purchase with your own or transactional funds and avoid the selling without a license question, and avoid assignment and disclosure of your spread or assignment fee. If this is your preferred route the obvious downside is that there are fees involved with transactional funding or opportunity cost if you are using your own funds.