I'd tell them sorry I don't know of anyone and wish them the best.  If it's out of my area I'd tell them sorry I don't work that area and wish them the best.  Goal is to be off the phone in 15 seconds or less.

"Will you cooperate with my office at X% of the purchase price?"
Yes? Good, send me the information.
No? Sorry, can't help you.

Once it is listed let me know BUT until then please quit wasting my time.

There best bet would be to go where the 'agents are', the MLS. Phone calls just won't cut it especially an entire state.

Yes, indeed I know many investment buyers for all kinds of properties. I wish you all the best with your survey! 

Sure, we get these kinds of calls all the time, however you will get more money for your property if you put it out on the mls so that ALL of the investors get a chance to take a look at it.

However if you insist on keeping it off the market you will likely sell it at a discount to a birddogger who will tie it up while they remarket it to their portfolio of real investors.

"Of course, I know someone. More than one."
My resources are not free. Actually, going 'outside' the system will be MORE expensive. In this described scenario a $10,000 introduction fee seems appropriate. Such a caller is looking for a chump.

Simple answer might be "We work with clients. Clients are those who sign agreements with us and then we do the appropriate research on their behalf. There's a good chance someone in our network might have a good lead in regard to the property you are offering." Something like that.

Buying, selling or investing is not complicated and requires minimum diligence, qualifications and action to succeed. I remain weary with people who try to reinvent the wheel in established Real Estate arenas. Conventional, established ways prevail!

"I represent many investors that might be interested.  Would you allow me to represent you should I bring you a buyer?"

I figure I only get those calls and calls for fix and flip homes also.

I do get these calls and these types of email. In 30 sec or less i can have the tax card pulled up. See if the name of owner and seller match and if so, get some info. I have actually sold a couple this way and with one guy, he now lists all his flips with me. 

Not in my arena, but appreciate the call.