The auction Item ID is a text (alphanumeric) value; therefore, numbers are treated as text and will sort differently than numeric values. To have the items sort in what looks to be numeric order, use a leading zero or zeroes.

For example, if items are being numbered 1-20, use one leading zero for 1-9 (01, 02, 03, 09, etc.). If items are being numbered 1-200, use two leading zeroes and then one leading zero for 1-99 (001, 002, 003, 010, 011, 012, 099, etc.).

If alpha characters are also used in the item ID, an example would be something like ABC-001, ABC-002, ABC-098, ABC-099, ABC-100.

On the auction page Auction Settings step in the Auction Information section, "Default Auction Items Sort Order" can be changed to Item Number.