We've all heard how important first impressions are but did you know you're making that first impression before you even open your mouth? This is the same with anyone you meet, not just buyers and sellers.
Next time you're meeting someone for the first time or even just seeing someone you've never seen before, take special notice of how you start forming opinions of them (albeit unconsciously) as soon as they catch your eye. The first few things you notice are appearance, motion, and energy. You'll notice how they're dressed, how their hair is styled, how they move through the room and if that's graceful, clumsy, or just average, whether or not they talk with their hands, the expression on their face, whether they seem happy, sad, ambivalent, angry, whether they're alone or mingling, etc. If you're at a networking event, you'll notice people before you meet them, and this opinion you form can make you decide whether or not you approach this person and introduce yourself.
Everything you notice about others is exactly what others will notice about you. Keeping a positive frame of mind, a smile on your face, and wearing clothes that make you feel great will create a great response in others. Doing things after meeting people like speaking animatedly, maintaining a healthy and genuine (i.e. not fake) smile, listening to and commenting upon what they're saying, and in networking situations... be sure to ask how you can help them rather than just telling them how they can help you.
Make sure you're the kind of person others want to be around and your network will do nothing but grow.