Why am I asked for source of funds?

The documents expected are concrete: a bank statement showing the original purchase of the assets, a sale contract, a payslip, a notarial deed. The aim is to establish a coherent link between the amounts converted and an identifiable source.

A second, separate check covers the on-chain origin of the deposited assets. It can produce false positives, since an address may have received dubious funds several hops upstream without its holder knowing. Manual review exists to separate those cases.

Preparing those documents in advance, when the amount you plan to sell warrants it, is the biggest time saving available across the whole journey.