With many players today treating Yugioh as a geek's version of the stock market or an independent business, it seems this game is becoming more like it's own economy rather than just a fun hobby.
People now look at your binder and give you the monetary value of your stuff, rather than just their own individual values. Why is this? First, we have to check the various factors that have been turning this game into it's own economic system...
Reason #1: The Secondary Market
This is pretty much the cancer that has spread through all card games (because it troubles many players). By having a secondary market, card games can become expensive or cheap depending on the gods of the market. However, nobody knows who is to control this monetary dictator.
The secondary market has been influencing many players in this game for quite some time now (perhaps during the ReturnDAD or Tele-DAD days, I forget). This has been causing a great divide between players, since now not only do we have a great rift between casual and competitive players, but we now have a rift between players with big wallets and those who are on a budget (this may differ for some people).
The wallet players are mostly known for having binders full of multiple copies of rare cards, decks made out of the highest rarity and complaining when reprints cause their cards to drop in value. Budget players are quite the opposite. They use the lowest rarity (not always, but it's common), are known to complain about the high rarities (let's be honest, who wouldn't complain...but just to be fair and neutral), but love reprints.
Wallet players are also known to run meta decks most of the time (some wallet players play other decks, but that's not an easy thing to see), while budget players will most likely be running anti-meta or any other deck that was once expensive but is now affordable (such as Glads and Blackwings for example), as well as non-mainstream decks.
You can tell who falls into which category by the way they ask you for trades, by their folder content, and sometimes their general attitude.
However, there's another aspect that turns Yugioh into an economy besides the secondary market...
Reason #2: It's not just a game, it's a way of life....literally!
Many players nowadays turn to this game as a main source of income. While others have allowances, jobs, or any other source of income, there are those players who make a living from selling cards alone. These are the Yugioh businessmen who are constantly seen handling money during trades, knowing the value of everything (and I mean everything), and gaining massive amounts of profit in methods unknown to many (whether they are clean or not, it depends on the person).
The players who depend on this game to make a living are mostly known for being paranoid around February/September, since their source of income may be affected. They are seen in tournaments looking to score a deal, and mostly are the leaders of their own Yugioh Mafia (other article needed for this one). They usually have others do their work for them, and some are rarely seen playing at all (not applying for everyone, since some play anyways). To some extent, they also enter tournaments to win (cleanly or underhanded is up to you to judge) and sell their prizes for cash, whether they be promos or boosters.
Reason #3: The Stock Market at YCS
Like most players in this game, YCS and other events turn into a guide into what the meta is and what players will be using. However, it's not just a guide to the meta, but a guide to some cash. By having different results in the YCS, the placements decks get will sometimes dictate the prize of specific cards in proportion to it's usage.
For example: X-Sabers once didn't cost as much since they didn't top major events, but once they managed to win, their prices skyrocketed and became a good investment.
By using tournaments as guides, players can know what cards to invest in. This gives them an edge in terms of profit, since they can help both the secondary market and their wallets. There are times when underdog decks rise to fame and become a good investment, by which will cause a rise in the YGO Stock Market which feels like a wave of chaos ensuing the players as they quickly swarm to the new cards for profit.
So whether you play for fun, fame or profit...one can't deny that this game has developed it's own economy which has now become part of the very game itself. Remember to keep your wits at all times and make the best decisions that fit your style best. Don't let this internal economy get the best of you, or else it will be quite the frustrating time.