Will the casters lock so the stool doesn't roll while I'm sitting on it?

Locking mechanism details are not specified — test the casters before committing to high-load work. If they roll freely under your weight, you may need to block the wheels with chocks or park against a wall.

Can I use this in a wet shop or does moisture damage the seat?

The seat surface is designed to resist oil and grease. For wet environments or washdown bays, dry it off after use to prevent standing water and potential rust on the caster hardware.

Is there a weight limit I should know about?

Weight capacity is not confirmed in the product specs — stick to casual diagnostic work and light component removal rather than extended heavy-load sessions.