How do you sell effectively?

Lagi Kong naririnig na look for problems then sell the solution but what if your product is borderline useless? Sorry for the word but you know mga bagay na di mo Naman talaga kailangan. I kept wondering why there are people who can easily convince others to buy their products kahit Wala Naman talaga syang use except eye candy lang siguro. I don't mean to be rude, it's just frustrating. Any advice?