When I first started trading it was all about matching edition size, an LE 100 for an LE 100 for example, or trading for what you wanted to collect. Since poking my head back in the game I've heard A LOT about 'Knowing the Trade value vs sale value of a pin' which apparently breaks down to this: People who are investing/using pins to do business are looking at trading a pin for a pin based on the highest perceived value, they will often drop that value when you are willing to buy the pin outright. Don't let that discourage you though, because I've met plenty of lovely pin sellers who need inventory and will happily work with you to 'trade up' multiple 'smaller value' pins for a 'high value' pin you might want.
@Sithathoriunet is right though, if a trade feels 'bad' just leave it, there are so many delightful people to trade with and so many pins that it isn't worth it to feel like you're being taken advantage of at any time.
There were sharks when I started, and apparently there are still sharks now.
(Don't know if the term is still used but sharks were what a lot of pin traders called unscrupulous traders who would take advantage of kids or people who were new to trading to get trades that were very one sided/unfavorable. I think Disney even released an official pin with Ariel wearing a lanyard and looking at a shark sort of scared.)