There seems to be a thriving business of buying foodstuffs at Costco and IKEA and reselling them. Even Nissin in Azabu-Juban resells Kirkland branded stuff at a considerable mark-up.

There was also a website that re-sells Costco stuff. In addition to things from Costco JP they also claim to stock items shipped from Costco in the US.

One thing about Costco is the time it takes to get there and tolls. For me it was maybe 20 min or less if the road is wide open and I could drive 100 km/h+) on the Shuto and tolls would be around 1600 yen round trip, so you have to buy a lot of crap to make up for that. Kawasaki was doable on surface roads but I would guess 40 minutes each way. When the final stretch of C2 opens then the drive should be much better. Or is it open yet? However, over 3 years and with a complimentary Costco card I never went there. I only once went to Costco in Chiba because I happened to be driving by.