If a product is out of stock on your site, immediately remove that product from your feed. If not, you’re potentially paying for clicks that have almost no chance of converting. If you’re a large merchant with abundant resources and approach the comparison shopping game from a branding/mindshare perspective, then go ahead and spend strategically.

If you’re a small merchant with limited resources, I wouldn’t even recommend using the handy ‘out of stock’ marketing message available on many engines…just remove that product from your feed or if you control the bidding through your feed, bid the SKU to $0.00.