Many investor-realtors eventually get their broker's license so they can be a one-person shop and work (mostly, or completely) for their own account.
Spouse is a broker, when we make an offer to a listing realtor who's new to us, he does a little explaining before making the offer. He explains what we do and that an owner occupant might offer more $, but we can close quickly and as-is. He stipulates up front that if the buyer wants to wait for a better offer, we understand - our offers tend to expire quickly, but several times the listing agent has called us back a few months later and asked us if we're still interested in making an offer. If it's a property that an owner occupant is going to want, we're likely not making an offer anyway. Also, if he is making an offer for a house we are buying, he usually foregoes the buyer agent commission - makes it clear how we intend to make our money.
Our first deal in Elmira the listing agent tried to talk us out of making an offer (too low, seller would never accept). Less than an hour later she got back to us - 'much to my surprise, the seller has accepted your offer'. Sometimes the seller hasn't told the listing agent everything.
Tone and explaining can help, but don't be afraid to insist that the listing agent presents the offer to their seller - at least in CT and NY, an agent is required to present all offers.
On the buyers-disappearing front, many folks think they want to buy but aren't actually ready. I would suggest interviewing them more carefully up front, laying out *your* expectations and a typical time line/activity level. That way if the buyers aren't serious, you can adjust your investment of time more quickly.