Satisfy your customer-
Above all else, try to keep your customer happy & satisfied. Humans aren't perfectly logical creatures- if someone like you, they're more likely to buy from you, regardless of the ideal you're offering.
Demonstrate your product/service's value- When it comes to convincing a relevant customer to make a purchase, glowing praise for your product or service is only going to get you so far. To really get the customer on your side, show your customer how the thing you're selling will make their life better.
Whether it will save them money & time, give them peace of mind or simply make them feel good, make sure your customer understands exactly how the item you're selling benefits him in real, practical forms.