Rich,
If they make a match that meets the criteria that you put into your search, then you have already agreed to accept that match. The more precise you are in your request, the harder it will be for them to march it for you because they have less inventory than the big 2. If they have something that is a close match, you will be asked if you want it, but you are not obligated to take it. Some people have been very exact in their request and they have been successful getting exchanges.
I have had very good luck getting what I wanted, but I am flexible. In August I took a vacation to Yellowstone. I had asked for an exchange in Montana. SFX called and said they had an opportunity at Island Park Village Resort in Idaho. I was not forced to take the exchange, but I decided to go for it because it was in August - prime time for a visit to Yellowstone. Some others might not have been as willing to take that resort so they could say no.
Sue