Not sure if I agree that a responding on Twitter would be SPAM!! It's not an opt-in service. The general expectation is that if you are posting on Twitter, there are ALOT of people who can see your posts and who may respond. So, as others have said, you can politely enter the conversation without barging in and saying BUY MY STUFF!! Ask questions. Find out why they are experiencing the pain they are. Find out what solutions they have tried. Find out what they thought of those solutions. Share things that you have tried and then finally share the solution that you have or are currently using that works for you. Maybe they will respond by starting to ask YOU a question or two. It takes time but its about relationship marketing.