And why did mangers bid on players for whatever prices listed? Because they thought their decisions were good of them. In your opinion, you think they are not worth the money, but apparently the managers who acquired them thought otherwise. Are they all stupid and only you're the wise guy? I don't think so.
Because eventually a sucker comes along.
The funny thing is that people used to complain about the daytraders in Hattrick back in the mid 2000s when it was truly an overpowered strategy there, but for the most part those guys were finding players on the TL who were actually worth less than their selling price and then would turn around and sell them at a normal to slightly inflated price. While I personally disdain the entire concept of treating the game as a market simulation where, from time to time, people actually bother with putting lineups, I can respect people with the discipline to find those good deals. What I find reprehensible is people who base their economy on fleecing people who don't know any better on players with players who aren't worth a third of what they're listed for.
Personally, I think that until you've had a player at least a season, the maximum you should be able to list him for is one-half his purchase price (with no limits for players you drafted).