When I sold floor coverings in a retail setting I sure wish I would have always had qualified, ready-to-buy prospects and no wishy-washing, sitting-on-the-fence ones. But it never happened that way.
You will have customers who are what I call 'matter-of-fact' buyers, who come in and look at products, learn a little about them, and say "let's do it." (Don't you just love those people?) But it's hard in the retail industy to only work with qualified buyers. And I suspect it's hard in any sales field to only spend your valuable time with 'buyers.'
Then you have the customers who are so afraid you are going to 'sell' them something something they really don't want, and they are scared to death they will make a big mistake. They come into your place of business with enough 'defense' to stop a good football running back.
But the fact they actually walked into your business means they need and want what you have to offer. If they didn't they wouldn't be there. It's your job to displace these fears by asking tons of questions, attempting to relate to them on a personal basis, and convincing them you are the expert in your field who is concerned with helping them get exactly the right product/service for them.
In retail, I would say 60-70% of the prospects come in on the 'defense.' I know when I go in to buy something I don't know much about I've got my own 'defensive line' with me and I'm not going to let this salesperson win the 'game.'
But when they ask me a lot questions about my situation and needs and wants, talk to me as though I'm a friend, and don't go on the 'offense' with me, I become more relaxed and open to listen to them about what they think would work best for me. When they sincerely help me get what I need and want, I buy.
These principles must be 'adapted' to other fields to work. When I sold real estate I made sure I 'qualified' the prospects to know if they could even get a mortgage. If they couldn't you would be wasting your time unless you wanted to work with them on a long-term basis and help them clear their finances to be able to get a loan.
It's always nice to work with only 'qualified' prospects, but in the 'real world' that doesn't happen every day. I think you should create plans and hone your skills to work with both, depending on your industry and what level of success you are at...