Hi Sigurd,
A 503 seems like an odd choice in this circumstance - is this something that the eCommerce software dictates, or are there options for this?
A 503 is service unavailable and isn't something I would expect to see on a page that has expired. There are a number of alternatives depending on the product & niche. For Google to be seeing thousands of 503's might make it look like the site is having problems.
If a page really must go, then it should be a 404 / 410, or a 301 to a close alternative, but not a 503.
Are there pointers that show these are causing issues with the site's SERP's?
-Andy