Lost volume seller is a legal term in the law of contracts.
The ordinary measure of damages fails to put the lost volume seller in as good a position as the lost volume seller would have been if the breaching buyer had been fully performed.
In such a case, some laws (including Article 2 of the Uniform Commercial Code in the United States) permit the aggrieved seller to recover the lost profit that the seller would have made if the buyer had not breached (plus, under some circumstances, incidental damages).
Lost volume sellers tend to be those whose capacity to sell or to produce or acquire the subject goods is sufficiently large to meet the demands of all customers who seek to buy those goods.
For example, if a T-shirt maker has 100 T-shirts, and a customer agrees to buy a shirt from the seller and then breaches that agreement (i.e. refuses to take delivery and pay), the seller will likely be able sell the shirt at the same price to the next customer who walks in.