Trading with guests, on the other hand, is very different. Cast members are required to trade pins on a one (1) for one (1) ratio. Because guests are trading with their own pins, they can ask for whatever they want in trade, regardless of whether it's fair. It's up to you to determine whether the trade the other person is offering you is fair to you. And this is the tricky part: pin values ultimately are highly subjective. At the end of the day, a pin only is worth what a person is willing to give up, whether in pins or monies, to acquire it. So how do you actually trade?
The best thing you can do is educate yourself on the value of both your pins and the ones for which you want to trade. Know how much you have invested in your pins. Look at how much they have sold for recently; I look at the last few month's of completed sales, discard the highest (desperate buyers who would overpay), the lowest (poorly listed auctions), and average what's left to give me what I consider the most accurate, and also factor in how badly I want it and what I would give up to get it. And I also use an enormous amount of common sense; if something seems too good to be true, it probably is. Ask questions; if the trader is getting defensive, there's probably a reason and it likely isn't good. A good trader will want you to be comfortable and happy with the trade.
And just like trading with cast members, you don't have to accept every trade offer that comes your way. A polite 'no thank you' is just as acceptable an answer as a 'yes, I would like to make this trade, please.' And an honest, respectable pin trader will accept that 'no' also is a complete sentence and not push.
And once you're comfortable with trading in person, you'll find that trading online really isn't that different. The biggest difference is that you don't see the physical pins in advance. But you can ask for detailed photos before agreeing to anything and it's poor etiquette not to disclose any significant flaws, so that's not really an issue. But the negotiation is exactly the same.