I'm sorry, how is capital gains tax different from "seizing of assets from wealthy people" again? It seems like a distinction without a difference.
Yes, technically the government is inserting itself into the transaction and seizing some of the value. But that's what all taxes do.
The government is also the party that is responsible for enforcing the terms of the transaction (via the justice system) if one party where to try to defraud the other. In effect it's more like a transaction fee.