Liking a post on Bunz can mean different things to different people. When you like an item it's saved to your profile and can been seen publicly under your 'Likes'. A notification is also sent to the owner of the post, letting them know you liked their item.
Most people like items they want, which lets the poster know to check out their profile. However, a lot of people like posts simply because they like the item, but don't necessarily want to trade for it. At the end of the day, there is no 'proper' way to use likes so use them however you see fit!