No, the idea is not ludicrous, but there are a few things you're going to want to know first, and a realtor can help you. Why are the people selling, have they purchased something else already, and how much do they owe? If it is a distressed seller and they have equity, then no problem. If no equity, then they won't be able to give the property away. If they've purchased another home and the home has been on the market for a while, they will also be more willing to deal. And how much they owe can usually be determined pretty close from tax records - also accessible to your realtor. And perhaps the most important question - how much is the home actually worth? If somebody is already 20% below market, maybe just come in a little under. There are still plenty of sellers that are easily 20 - 25% too high, so you wouldn't want to pay them 75% of their asking price.
In short, many realtors don't mind low-balling the seller, but sellers may get angry and then you're not going to be able to revisit them again later. As long as you're comfortable with that, go for it.
If you want a referral for an agent in your area, let me know. Easily done....