What do you do with people who promise, promise, promise that they'll order from you, but just keep asking for more samples and additional time? Just because you're a salesperson, doesn't mean you're a doormat. If you feel like you're being used, you probably are. You need to deal straight on with your prospect and say something like this:
"John, you have said a gazillion times that you're going to order from me. You ask for tons of samples. I quickly respond and get you what you need. But nothing happens. While I'd really love to do work with your company, it can't keep going on like this. So what do you want to do? Should we get you going? If so, I'd recommend…"
Say it nicely but firmly. It is simply a statement. You're not blaming or whining. It's a simple statement of fact. If he makes one more request, politely tell him that you will honor that, but then it's over. After that, he'll either need to place an order with you or you'll stop bothering him.