When there are lots of people looking for homes but not many for sale, it's called a 'seller's market'. When there are lots of homes for sale and not many people buying them, it's called a 'buyer's market'.
If you're selling one home and buying another, you don't really have to worry about playing the market. If you sell your existing home for a 'low' price, you're probably also buying at a low price.
It's true. Winter sales tend to be slower, and spring sales are more brisk.