If you're really hungry, perhaps any buyer with a pulse is worth your time and resources. However, many an agent has been burned out early because they used up their money, time and enthusiasm on buyer prospects who were not motivated at all. It's OK to be selective in your business as to who you work with.
Time frames stretch, as internet buyers start their searches much earlier and spend more time in research. If you dismiss or procrastinate in serving a web buyer prospect because they say they will be buying in a "year or so", you'll probably not work with many. You'll also lose a lot of business in the future.
Proper information delivery systems via email and the web should be set up so that you can work with these buyers over time.
If you work in a market with a lot of first-time buyers, you will be well-served to try to get them to get a mortgage pre-qualification letter. They might not even know what they can or cannot afford.
If you work in a resort, vacation or luxury market, you may not want to take that approach for fear of annoying the buyers. You might find an internet search and polite questions about their business helpful.
Asking questions and getting them comfortable with you might clarify their intentions. If it's clear to you that they have no intention of making a purchase in the near future, you need to decide how much time you want to spend with them. Though you may lose them, you can print up listings for them to drive by or do open houses.
Perhaps your web site gets a prospect to you that wants property in your MLS, but a 50 minute drive across town, you may find it better for your time and expense to refer them to an agent in that area. Perhaps they're looking for commercial property and you're inexperienced in that area. It would definitely be better to refer them out in this case.
These could be clients who will end up plaintiffs. If you're getting a bad feeling, pay attention to it, as it could keep you out of court in the future.

