I don't find that transaction costs / liquidity story very convincing. Buying from someone who knows something is no more damaging than buying from someone who doesn't if its the same stock at the same time.
Suppose someone is selling you a house. They're desperate to sell : are you saying it makes no difference whether they're selling because their aunt just died (i.e. random sale) or they're someone within the town planning department?
Or suppose it's a car mechanic that wants to unload a car cheaply?
Or suppose it's a car mechanic that wants to unload a car cheaply?