Liquid goods are easy to buy and sell. Illiquid goods are hard to buy and sell.
I think we need to break up "buy and sell" into its two component parts.
In a buyer's market, goods are easy to buy and hard to sell. The same good is liquid from the buyer's perspective, and illiquid from the seller's perspective.
In a seller's market, goods are hard to buy and easy to sell. The same good is liquid from the seller's perspective and illiquid from the buyer's perspective.
I think it is true that buyer's liquidity and seller's liquidity vary inversely over the trade cycle. In a recession, buyer's liquidity rises and seller's liquidity falls. Recessions tend to be a buyer's market for normally illiquid goods. In a boom, seller's liquidity rises and buyer's liquidity falls. Booms tend to be a seller's market for normally illiquid goods.
If I am right about that stylised fact, it seems unwise to ignore it, when we build theories of the trade cycle. [Update: It's a large part of what we are trying to explain.]