Yes agree to most except for court .The lawyer I know wins more then he looses and he’d handle it for me so my cost would be what $500 to file.You're right. They could and, under certain circumstances would estimate your original cost, and calculate your gain from there. But, unless they can prove easily and exactly what the current selling price was, and the price was very, very large there simply isn't enough tax revenue in it for them to bother with 99.999% of the personal stuff that gets sold. Hell they're still trying to get their cut of tax on all the sales of land, camps and cottages that change hands every year let alone worry about a couple guns or the odd painting.
As to tax court, sadly if they decide to fight you in court, you lose. Maybe you win your case but lose in legal costs. The CRA has far deeper pockets than any ordinary citizen so you would be far better off to pay any assessed tax and move on if it came to that. But, as you point out, they don't really want to go to court over small stuff. No money in it.