If you get turned down for being overqualified, it usually means that the store is looking for part time workers - but not the ones who are going to quickly jump to a new job as soon as they get trained. If their going to put the effort into training you, they want to be able to rely on you not to run off which seems like a fair point. If they employ people who aren't as qualified, who aren't as liking to get a better job, they'll get reliability. Students are good because while their at uni during the "quieter" period they don't have to pay them and when they come back for the busier holiday peroid they don't have to employ and train new staff members.