Not less weird at all, it's very weird to them, and precisely because it's traceable, you must prove it, if you can't prove it, then how are you supposed to sell?
Again, suppose that I sent coins into Mintpal, traded 1 BTC for LTC, and sold this LTC when it went up for a profit of 4 BTC, I end up with 5 BTC, then I deposit this 5 BTC back into my wallet.
Mintpal dies out of nowhere and now you don't have your account anymore. How do you prove the origin? and if you can't prove the origin, you are fucked. This is why im scared to sell BTC. Yes you may be able to solve it with signature campaigns (and im still worried that I should have been paying taxes because it would be considered a wage in exchange of a work and not an investment). They may decide to tax you with an extra for all the months that you didn't pay taxes on what is considered a wage when they realize you had a lot of payments (because once they have your bitcointalk account, they can trace all of your payments ever here, not just what you are reporting).
You have chosen the wrong example.
You said that you have sent coins to Mintpal, 4 of them
So , here we have the starting point.
Then you traded on Mintpal and you withdraw 5
BTC.
Really don't see the problem here.
In case your country has laws for this in place you're going to have to pay for the profit you have made from trading.
It will help us a lot if we discussed with clear numbers and location of the activity (your country).
PS.
This may be ok if you are just dealing with small amount of sales, but let's say in 10 years 1 BTC is worth $1,000,000
How do you cash out $1,000,000 or any other amount near that? you can't do it through localbitcoins, you would need a million different merchants to cash out that much money.
Don't worry, in this case you could afford the best lawyers out there so you could probably get away with murder, not just taxa evasion.