The single most important thing you must learn about yourself is what kind of Magic player you are. This will determine what your trading guidelines and goals look like. For example, if you are a "Spike" player, one whose only goal is to compete at the highest possible level by any means possible, your focus will be on acquiring all the cards you might need for upcoming tournaments. If you're a "Timmy" player, one who plays casually with friends and enjoys doing big, bombastic things that may or may not lead to winning the game, your goal will probably be to acquire cards that simply entertain you. There is no validity or lack thereof when it comes to one's card acquisition goals. There's no right or wrong. For the vast majority of us, this is a game, and whatever you need to do to make it fun, that's what you should do.
The ethical dilemma occurs when your goals don't coincide with the goals of the guy you're trading with. What happens when you want a Fetch Land ($15+) and the other guys wants an Iona, Shield of Emeria ($6), and he calls it a fair deal? Do you take it?
Stop and ask yourself what you'd do in this situation. Nothing further has been said. He pulls out an Iona, you pull out a Misty Rainforest, and he says "Sounds good to me!" What do you do?
Now, there really isn't a right answer. If you're a tournament player or someone who just enjoys building a collection through smart trading, you'll likely accept that offer. Here's where a ton of other factors come in to play (am I supposed to say "enter the battlefield? That feels wrong.) Does this guy know you're almost tripling up on value? Does he care? Does he have any idea why fetchlands are good? Is he a buddy? Does he play at the same shop you do? Are you ever going to see him again? These are all things to consider.
Now, be warned that this part may be a little Machiavellian for the tastes of some readers. Let's go through the questions one-by-one. First, does your partner realize the value of his cards? If not, then you're taking advantage of someone. That's generally considered to be wrong and not a nice thing to do. The counter-argument is that we DO live in an age where the internet is available almost everywhere, and in many cases, card prices are but a click away.
It is honestly the responsibility of everyone who spends time and money on a CCG to understand the value of their cards. It is also the responsibility of someone who does NOT know the value of their cards to find out, or ask. If you don't know the value of your cards, there is no excuse when you get ripped off. We'll get to the part about why ripping people off is bad in a second, but just do your bit and learn the value of your cards to avoid the issue entirely.
Sometimes, people just don't care about "book value". These people are great to trade with because as someone who DOES care about book value, you will almost always get a great deal. If they are aware of the price and don't mind doing the deal anyway, then you're honestly doing nothing wrong. Remember, just because someone else's goals are different doesn't make their goals any less valid. Your goal when trading is to turn a profit. Theirs probably isn't. That's ethical and everyone leaves happy.
Now, lets talk about why its bad from a financial standpoint to rip people off. It's difficult to discuss ethics because everyone has their own comfort levels and definition of good and bad. Myself, I consider myself an ethical person who tries to do good and help people out when I can, but be aware that not everyone is as altruistic. There's an old saying, "You can shear a sheep many times, but you can only skin it once." Maintain a good business relationship with your trading partners and it will pay off in time. People will be keen to lend you cards, take things "on credit", and will often times cut you a better deal because they know you'll be honest with them. In business terms, it's better to never burn a bridge. The few dollars you make off one skewed trade won't amount to anything in the long run, but the ecosystem of friends and allies you build as you play in a community will pay itself off in spades.
Just think of it this way: next time you need a ride to a PTQ, or to borrow a set of lands for your FNM deck, or the next time you just need someone to spot you a buck to buy a soda, who's more likely to do that? The guy who you screwed out of a Maelstrom Pulse or the guy who you do fair, balanced deals with every week? Yep, that's an easy one. The power of a social network is beyond compare, and it would be wise to maintain that network like you'd maintain a classic automobile.
A lot of people hear that and they ask, "But what about at a big event, far from home? What then? NOW can I go rip people off with my financial savvy and make tons of money?" Sure, go for it. But know this - ethics are what you would do if no one else was watching. Would you take an unattended deck box if you knew you wouldn't get caught? If so, maybe your mother should have a talk with you, because that's called stealing and it's wrong. It's just the same as trading with someone you see often. Treat everyone well, with respect, and make sure they at least feel like they know what's going on in a trade, no matter who they are. They could become the next link in your social network, or heaven forbid, a friend.
The bottom line is that everyone has their own trading strategies and values. A lot of value can be gained from leveraging these values against eachother. Knowing what cards appeal to which types of players is almost more important than knowing the actual prices of the cards. For example, Timmy loves big fat creatures with ridiculous effects - Iona, Shield of Emeria is just about perfect for those players. Johnny, on the other hand, loves weird techy cards that take a bit of work to get going. Mill cards, namely things like Archive Trap, and alternate win cards are hugely popular with Johnny. Spike loves efficiency and top-shelf design. If it's in a tier 1 deck or could be, Spike drools for it. Examples are cards like Goblin Guide, fetch lands, and Bloodghast. Know these players, know how to identify these kinds of cards, and learn how to use this knowledge to get a good deal that makes everyone happy.
In the end, if both parties are fully informed and happy, then the trade's good. It doesn't matter what the book value of a card is if neither person cares. Likewise, it makes no difference how neat Felidar Sovereign looks if both players are trying to finish their Jund Cascade decks for the PTQ in 12 hours. It's perfectly reasonable to walk away from a trade if the value systems are too far out of synch (or too close!). You're never under obligation to make a trade with anyone, and it never pays to get upset about a trade falling through. Just keep your trades fair, keep everyone informed, and make sure everyone leaves happy and your binder will always be full.