This wouldn't be that cost-effective...

Nordstrom gift certificates sell at about 85-93% of face value.

So buy $5000 in gift cards = 20,000 miles.

Resell $5000 at 89% of face value (without any listing fees added!) = $4450.

Net cost of miles: $550.

CPM: $550/20000 = 2.75...

You can do better than 2.75 on a mileage run, not to mention the fact that this is based on the speculatory ability to sell these gift cards on ebay, and you will likely saturate the ebay market and lower the selling cost...

Better to fly and earn EQM if you have the time.