Every time I get to the point of making a contract on a deal, the seller backs out and says the don't want to constricted to just one buyer. Am i doing something wrong?
Ask why? Press them for an answer.
Most likely if you are getting the same objection, it's in your setup. You aren't getting a crucial part of the buy in. Look at your sales script, and see where there might be some mention of only one buyer.
The objection can probably be overcome with a simple "There can be only one buyer for your property, and he's sitting right here in front of you?" followed by, now, we agreed to $xxx dollars, correct. Then take them through the close again.
Without seeing your script, those are my best guesses. You could run it by a trusted, seasoned RE person to get a better take on it.