Dadbaur hit the nail on the head. The thing that most people don't fully grasp is that value is incredibly subjective. Just because the market values a card at some dollar value, does not mean that card is worth that to everyone. If I'm not building a deck that needs that card, I certainly put very little value in it. Similarly, if there is a card that is in a deck that I am playing, or I need for a deck I want to play, I probably value it above what the market currently has it at as far as trading is concerned at least.
Everyone most likely understands this in the form of cards that are "for trade" and "not for trade", however reducing this to a binary option massively over-simplifies things. If I offer you power for smaller cards that are in your "not for trade" pile, are you going to say no? Almost certainly not. So "not for trade" is not really that, it just means you value those cards enough above market that it's unlikely someone will make you an offer you are interested in. At some point though, everything has a price. For each card, and person, that is going to be different. As such it's not at all unreasonable to assign values to cards in a tradelist or wish list that differ from their market value. Cards that are in high demand, namely duals, fetches, shocks, etc. have an intangible value in that everyone wants them, so there is a high likelihood that when you find someone that has something you really want or need, you can trade these staples for them. Consequently it becomes foolish to trade away these for smaller things that are of lower priority, even if the "value" is even.
In order for a trade to happen, each person has to need or want something from the other. If I have card A, and want B, of equal value, and you have B, but want C of equal value, we can't make a trade happen. Let's add party 3, which has C, and wants A. Unless someone is incredibly astute, and takes note of all 3 parties, and arranges a 3 way trade, no one gets anywhere. Cards only actually have a value when you move them. If they're sitting in a binder, with no one interested in them, then they are effectively worthless. It's up to you what the cost of sitting on that card is waiting for the person that does actually want it, AND has something you want in exchange.
People that trade for value, don't actually need anything from you. They generally don't play, so they aren't trying to complete decks, or play sets, or things like that. There is literally zero reason for them to trade at even value, because trades carry cost, namely postage, and the inherent risk of lost mail or scam. So even trades are, in the long term, a financially losing proposition. It is of my opinion that these traders, similar to stores, actually offer a service to the community, in terms of facilitating liquidity. Without them, far fewer cards move, so it's very likely you end up sitting on cards that have "value", but not to the people that have what you want. I have absolutely traded piles of standard cards, and EDH cards that I am unlikely to ever use, or trade, into duals in the past, and happily gave up 30+% "value" for them.
That said, the few people that I have dealt with in this manner have made it abundantly clear up front that that was how they did business. It was either clearly stated at the start of their profile on here, or they said that before we opened binders, so there were no surprises. I knew what I was paying, I knew what I was getting. Citing some of the earlier examples in this thread, where 20 minutes into the trade someone pulls out the "oh I need some value" is super shady, and I would not be pleased to have that come out of the blue at me.
Long story short, if you want a card you have several options available to you, each with it's own cost.
1.) Buy the card, clear financial cost.
2.) Try to trade for the card evenly, which depending on what you're getting and have to offer, could be a long wait to find someone that needs exactly what you have, potentially a time cost. How badly do you need the card? How much is your time worth to you?
3.) Give up a little bit of financial value for simplicity, by offloading cards that you otherwise have difficulty moving, in exchange for cards that you otherwise have difficulty finding.
To say "it's unfair for some people to trade for profit" or "tradelist means for trade, wishlist means I want it" is an oversimplification that doesn't really do justice to all the many factors that play into how each person values their cards, time, and money.