If you really train them, then you are adding value to them. If you just sell them with no training why should you get more money for them? Try to get back to the tax topic, if you list your player for more than it's worth, even during the daytrading attempt, you should pay tax. But daytrading is another topic alltogether.
Take the scrimmage scrub as a $1k salary contract player. Just like a 10 day contract.
If you buy someone in average GS on Monday and list them to sell Thursday where they arrive at the new owner with perfect minutes and 48mins in a training position you are adding value to the player. Remember the 'day trader' takes him on in his current shape of poor GS and doesnt benefit from the minutes, he polishes him up for someone else to enjoy and benefit from.
My argument might seem far fetched but when you are running at a loss or the margins between success and failure are very thin (as they tend to be in pyramid style games) you again misunderestimate how some people will attempt to create slight advantages.
This in a way is no different to NT's trying to buy opposition NT players with farm teams and then ruin their GS. Why do they do it? Because they can and it might create a competitive advantage.
Back to the topic, you have expressed that you want a tax levied based on nothing other than a personal opinion that stems from your own disatisfaction of seeing players at prices you wouldnt even conceive.
If I buy a player on Monday for $100k and believe I've added sufficient value to justify listing for $150k and someone agrees great.. if their doesnt happen to be a suitable buyer for my listed guy that week - I suck up the wage on the economic update and I can try again the following week.
Maybe I should ask to impose a tax on you for buying a $1k guy and then dumping him.. perhaps whenever you sack a player within 1 month of buying him you pay 20x his wage as severence for ending his career? Perhaps this would stop people treating scrimmages so lightly?