Sale spoiling
Please do not march into a thread and state that you can get an item for $X on eBay or some other online store. If you think their price may be too high, don't be rude about it. If others feel the same way, then it won't sell. This is a hobby where each person gives an item a different monetary worth. While it's fine to look out for others, people should also do research into the item they are buying.
Similarly, if you want an item that someone else is also asking for, it is better etiquette that you create your own topic with your contact information, trade list, or the price you're willing to pay.