There is no way to "convince" someone to buy a .net name, as far as a methodology that is somehow unique to .net itself.
They're either interested, or they aren't. If they're interested, it will generally because you can offer them very high quality keywords that are otherwise unattainable in .com. If they're not interested in your .net name, it's either because your keywords don't appeal to them, or .net doesn't appeal to them.
If you find yourself 'making a case for .net', you aren't selling right.
I own (and love) .us names. Like, really, really good ones. No garbage. From time to time (although pretty infrequently) I resell a .us name or two. When I pitch a .us name to end-users, they either 'get it' immediately on .us, or they will never 'get it' at all, no matter how I explain it to them. Entering into a sales pitch by immediately launching into apologetics for what you're selling (.net is JUST AS GOOD AS .COM!! IT'S JUST THAT THE REST OF THE WORLD DOESN'T REALIZE IT!!!) is an awful, awful strategy.
Your .net will sell on the strength of its keywords- not the fact that it is or isn't a .net.