Selling radio by qualifying - not "selling"
In my experience with clients looking to advertise, I wouldn't say there's been any fear of radio, especially since the price of radio is so much cheaper than TV. People look at radio as the cost-effective alternative. I would caution people against things like having the station do the ad themselves and accepting the schedule put together by the rep.
I think, however, the advice I would give you would be to NOT fear asking the question of your prospective clients. That's great that you can see their body language. Now take advantage of it.
Have their own interests completely in mind, much like an earlier post that said sometimes their advice is to not recommend radio in the proper context. I think that if your prospects see you motivated primarily to help them get more customers - regardless of the means - rather than motivated to sell them radio advertising, their resistance to you will start to melt.
As a sales pro, I have a bit of a contrarian point of view on sales. I do NOT think that the primary focus should ever be on selling your product. Rather, I think it should be upon learning about your prospect/customer, discovering their issues/problems/pains/fears and discussing the means for eliminating them. If your product does that, talk about it. If it doesn't, thank them for their time and consider yourself lucky for not having sold them something that won't work for them.
So perhaps the next time you see someone's body language say, "I don't want to be sold radio advertising by this person", ask them a question: "I'm always interested - What has been the most effective means yet that you've used to find new customers?". Or say, "You probably think I'm just here to sell you radio spots. I'm actually most interested in learning from you briefly about what has worked for you in finding new customers." If they are confused by that question or statement because they have been trained to think all salespeople just want to cram something down their throat, tell them that radio isn't for everyone, and you're more interested in finding out about their needs than just selling them a bunch of radio time. You'll be glad to get them on the radio if it is right for them.
If this makes sense to you, let me know how it works!