If you give demonstrations of your products or services then I recommend that you give this closing line a try:
“Why don’t you give it a try?”
“Try” is a very powerful word in sales as it seems to reduce the risk of the purchase to the buyer.
“Why don’t you buy it?” may seem a little heavy handed to some purchasers and may seem a little front on.
Whereas the word “Try” sinks into our minds as though there are no long term commitments and that we’re “having a go” with the product or service to see how we get on with it rather than purchasing it although you are purchasing it!
Why don’t you give it a try? 
Happy selling
Sean
Sean McPheat
Managing Director
MTD Sales Training
Download some useful resources:
Sean McPheat’s latest CD Sales Programme