Not Available in Stores
Usually during infomercials, the salesperson will mention that their item is "Not Available in Stores!" The intended implication is that this product is too exclusive to be sold just anywhere, but it often makes the sellers look so cheap that no respectable retailer will ever deal with them. It can also fool the consumer (though this hardly works) into believing the product being sold will never appear in a store, pressuring them to call right now and buy the product.
There is a chain of stores in American malls called "As Seen on TV!" that specializes in selling products like these; therefore, the claim is technically a lie. In fact, many products that supposedly are not being sold in stores will be sitting on department store shelves a week after the commercial airs. Interestingly, said products tend to be placed in their own "As seen on tv" section instead of the appropriate department, as if the store doesn't want shoppers to mistake a knife that "Never Needs Sharpening" for legitimate cutlery.
A common tactic in many infomercials or advertisements is to state instead that "this offer is not available in stores," (so call now, Operators Are Standing By) usually in combination with some form of But Wait! There's More! That way, the product itself can be found in any number of stores, but the statement is still technically true.
So why do they advertise this way? Because they get more money if you order directly from them, eliminating the middle-man. The maker of a new product might even desperately need this money for seed capital to get the product into stores.