It'd be very annoying to go to Costco there, return to the airport and return to town. Ilsan is in Northwest Seoul suburbs. The bus runs every 20-30 or so and it is a faster bus (just three stops airport to costco).

But, if you went to costco, went back to the airport, that'd take a minimum of two hours, so realistically three to pad yourself. Then another hour to get into Seoul and another hour back to Seoul by train.

When you're in Seoul, just go to Lotte Mart at Seoul Station. It's a big Wal Mart/Target esque type store. All the same stuff you get at Costco, you can get there (in much more manageable sizes).

All subway stations have lockers, but their location, number and availability of course, vary.