These are the consequences of wearing used…

In response, the skin thickens to protect itself, forming a hard, raised area that often appears yellow because of a buildup of dead skin cells (keratin). Hard corns usually form on the tops or sides of the toes and may have a central core that feels painful when pressed, often described as feeling like stepping on a small pebble.

Corns/ NHS
Other Possible Causes (Less Common)

Blisters: Prolonged friction may initially result in blisters full of fluid, which may have a yellow appearance if they get infected.

Fungal infections: If you slip on a pair of previously owned shoes, you could be inviting fungi that cause athlete’s foot and nail infections to set up home on your feet. These infections usually present as itching, redness, peeling skin, or yellowed nails rather than a nodular, firm lesion.

Why Second-Hand Shoes Are More Likely to Cause Problems