Check for out of Stock Merchants
It is obvious you should regularly check the merchants you are linking to have the items in stock you are promoting.
However it is something that I overlooked and after going over the stats for yesterday I saw quite a lot of the items I was advertising on my sites were out of stock. Some of them were still displaying as in stock on my Easy Content Units (ECU) but when I clicked through to the merchant the items were no longer in stock.
Chances are the user will go back to your site and try another merchant but there is also the chance they will just load Google in the current window and find another site with the item by-passing your site.
Another issue I had was that a few merchants had gone out of stock and Play.com became the lowest priced merchant with the item in stock. This was bad news as Play.com seems to only pay about 1% commission so suddenly my sales were at a similar level but commission was slashed dramatically.
All the above would apply to SEO driven sites but with PPC it becomes much more urgent as you are paying for all your traffic and a bad day could wipe out all profits.
Again I guess this highlights the problem of having lots of small sites that applies to the approach I have taken so far. The more small sites you have promoting individual items the more site and products you have to check to see if everything is working.
I think once the Christmas traffic has died down I will try and setup up a hand-full of high value PPC campaigns that get me £20 or so commission per sale. Hopefully this should be easier to manage on a day to day basis.

You can use http://www.azgizmo.com to monitor out of stock items at Amazon. The site gives you an INSTANT notification when the inventory is loaded on the website. Good points: 1) It’s FAST – checks the site every 5 seconds! 2) It’s FREE 3) Auto-links you to the product site so you can GRAB it quickly – and alerts you with an alarm! 4) Helps you save by ONLY buying Amazon products instead of overpriced third-party items.
One other thing you have to look out for is that a product will show as being out of stock but may be back in stock, you have to remove the out of stock one and do a new search. That happens with John Lewis and probably others too.
Thanks for that, I will have a look.
Oh dear, sounds like I better keep a closer eye on things.