The 1/3 upfront would certainly separate the tire-kickers - but how do you determine exactly what 1/3 is since there is no ratified sales price? Look up small claims codes and go by that? Don't know if a judge would like that idea; better to have an already established amount ($1,000, $2,000, whatever) so there is no confusion. I would think where there is doubt, to be equitable, a judge will side for the underdog.
Many first-timers barely have enough funds for an FHA downpayment. I would require a retainer fee (maybe $500?) upfront for this person (it can be credited towards any cooperating broker commission at settlement if they do purchase a home - if that relieves your conscience some). I've never asked for one myself yet; although I do have a penalty in my listing agreement if a seller wants to end the agreement before the expiration date. But, a buyer retainer won't be far behind b/c it's become too costly being in business lately.
Our standard Buyer Representation Agreement has a section where you can add a retainer or whatever you want to call it.
But, what if there really isn't a "good" home available yet for the buyer? Sometimes it takes awhile for the right home to come along; but, those who are
always offering lowball prices and never accepting a counter, in my opinion, aren't really serious because they can always back out of the deal if they have a home inspection and use that as their parachute. Their hoping to buy the world on a dime.
I think many of us have had a client or two that we were relieved to see go. I had one who I worked with for several months. I was so sick of her. I stopped calling her, but, I see she did buy a home two months after that, but not through me [thank goodness].
I have an "invester" wannabe who keeps emailing me about cheap, cheap listings; asking me to "research" his questions - he hasn't taken the hint yet - I don't work for free (and there are bigger fish out there to be caught). In a day or two, I'll email him the facts of life and real estate - the retainer fee - that should tell me something.
We're human beings and even we can only tolerate so much.

Perhaps we should take a poll of how many had a client who purchased a home through them but who never, ever want to hear from them again - not even to sell their home!