Those sound like parts of the same thing to me.
Sure the seller gets more as the market goes up, but in a steadily rising market it means nothing to get more because you can do less with your money. This leaves us back in the original scenario of playing the market becoming the dominant strategy.
Your numbers of 70-80% and 80-100% demonstrate the problem exactly. In a stable market product is sold at a relatively constant rate. There can be surges, and fluctuations, but those don't really matter at this point.
When day traders will bid up to 80% on a player 80% becomes the starting point for regular bidders. This limits the number of players that will be sold, as permanent players, below the 100% threshold because once an auction starts there is only 20% of value left to go before you have crossed it. That value you gave of 80% is the key. If day traders stopped at even 50% the entire scenario might be different, but you can hardly ask someone to limit himself that way, it wouldn't be practical.
If the number of trades allowable is limited then less players will be pushed automatically to that 80% point and the market has a chance to level off at values nearer to what supply/demand ratios would predict.
In short: If I want a player, I know that traders will autmatically push his price to 80% of what I should expect to pay for him. Once that point is reached I have to then bid against any one else who wants that player. This invariable pushes prices over the threshold.
Markets trend in the direction they are already moving without outside influences. If players were regularly available at 80-100% of market then prices would be trending downwards. Instead they are rising, and this is because of inflation due to massive market speculation (ie: day trading) pushing prices up to at least 110-120% and at times even higher.
No, buying a player at 80-100% would not be a bad deal, but that doesn't happen because the regular bidding starts so high.