One of the biggest mistakes people make in sales is that they oversell. If you do this, your buyer will think that there is something wrong with your product or that you are desperate to make the sale; caveat emptor. Once you think that the buyer wants the product then stop talking. 

If you oversell, then the buyer might see blood in the water and talk your pricing point down and destroy your margins or commissions. More often than not, they will simply not buy your product. 

However, what do you do if you have no idea if the sale has closed? The best salespeople are relentless until the customer expresses interest in the product they are selling or, more importantly, until they hear no.

You have not failed at selling your product or service until the prospect says no. Until then, keep trying different angles to sell your product or service. Be creative and don't ever give up on this lead until she or he says no. Until then, all you heard was "not no".  

It is also imperative to bring up the largest risk with your product within the first 5 minutes of pitching it to prospects. The rationale is that it is the ethical thing to do and your potential customer will likely uncover the risk as they do due diligence on the product; transparency builds trust

It's crucial to disclose all other significant risks to your clients or prospective clients before doing business with them. The only reason you might not offer 100% disclosure is in the rare situation when you can't divulge an issue due to confidentiality reasons. If this is the case and if the investment or product/service has significant risks that outweigh the potential returns, then don't sell the product/service.

In business, relationships are always more important than product knowledge. It doesn't matter how good of a salesperson you are if your prospect doesn't trust you. Life is too short to destroy your or your company's reputation and compromise your values. I love Warren Buffett's quote of "It takes 20 years to build a reputation and five minutes to ruin it. If you think about that, you'll do things differently."

Remember that your customer needs your product and you are helping them by giving them the opportunity to purchase the product. The key is to never ever oversell. Once you sense that a potential customer is committed to purchasing your product, simply shut up. : )