Do not give your work away for free. If you are shooting work that people want to use, then you are meeting a commercial need. You don't mention what sort of requests you get (are they individuals, companies or publishers?) so its hard to be specific about why you don't hear back.
Sometimes people are just "trying it on" to see if they can get them for free. Eventually they will find a photographer who says "yes", either because they do not appreciate the commercial benefit to the user, think their work has no $$ value, or are just flattered to be asked. Declining to give them for free has helped no end. It sets a standard so when someone says "we have no budget" then they understand they get "no photos". Sometimes those who claim zero or small budgets can find money when they really need something...
As for theft, next time a publisher advises they will lift images off the web for a their book, remind them they are infringing copyright protected material and if they do you will seek to recover damages.
As for selling your own work, identify who might want to use your images (magazines, newspapers, whatever) and pick the phone up and call the editor. Its a hard business and can be soul destroying to hear "no thanks" 10 times in one day....but understanding the market wants/needs plus persistence will mean a sale.
I have just delivered a piece about copyright to Peter Rowlands at UWP mag and I think its due out next issue. There is much more advice in there than I can possibly cover here so keep a look out for it.
Good luck
Simon Brown
www.simonbrownimages.com