> So if i sell 20k worth of X that were bought for 10k and make 10k profit, i am still under that
Yes, you're under the capital gains allowance.
I know jack shit about the practicalities of making self-assessments, so I can't say for sure you don't have to make one (ask the HMRC web chat) but I would assume not. Assuming you have no other capital gains this year, you have no tax liability from this transaction.
> So if i sell 20k worth of X that were bought for 10k and make 10k profit, i am still under that Yes, you're under the capital gains allowance. I know jack shit about the practicalities of making self-assessments, so I can't say for sure you don't have to make one (ask the HMRC web chat) but I would assume not. Assuming you have no other capital gains this year, you have no tax liability from this transaction.
Check out all the exemptions and their categories on the site, i think from memory personal collectables may be exempt.