Just finished a loop for a junior buyer role in the Kirkland area. Three rounds total. Here's the breakdown.
Round 1 was a 30-min phone screen with a recruiter. Standard stuff: walk me through your background, why Costco, what do you know about the buying function. I'd done my homework on their supplier relationships and the Kirkland Signature strategy, and that clearly helped. The recruiter was warm but efficient.
Round 2 was the real one. Two-hour in-person with a regional merchandise manager and another buyer. Every single question was behavioral. I counted at least 8 STAR-format questions. The ones that came up: tell me about a time you pushed back on a supplier, tell me about a conflict with a colleague and how you resolved it, describe a time you made a purchasing decision with incomplete data. They wanted specifics. When I gave vague answers they'd ask 'what exactly did you do vs. the team.' Caught me a few times.
Round 3 was a 45-min call with a VP I barely caught the name of. More conversational but still behavioral. He kept asking what I'd do if I disagreed with a decision from above.
Overall: they seem to care a lot about judgment under uncertainty and whether you'll stay. Multiple people mentioned their low turnover rate. Take that cue. Total timeline was about 6 weeks from application to verbal offer.