My thoughts mirror @wfo's, but more simplistically. It is a negotiation tactic. When seller A wants to sell something for $20, it is initially offered for $30.

My thoughts mirror @wfo's, but more simplistically. It is a negotiation tactic. When seller A wants to sell something for $20, it is initially offered for $30. When buyer B's $25 counter offer is accepted, B is pleased with the result because they've purchased the item under asking price, while A is pleased for having sold at a premium.