If you sell a Monster and want to buy it back, your most recently sold Monster will appear as the Buyback option found at the end of the Monsters section in the Market. You can buy it back for the sell price, and it doesn't require incubation. Additionally, the Monster will be at the same level it was at before it was sold.

Keep in mind only Monsters that have been sold after being placed on the Island can be bought back. Eggs sold directly from the Nursery are permanently removed from the Island and cannot be replaced.