h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Are these completely prohibited or can you use them if they are in context with the entry and simply included as a 'further information' type thing and not a plug or advert?

The general rules I follow - which I believe I heard from an Italic when I first started Aceing or Subbing (so probably one that has left) are:

If it has a shopping basket on it then don't link.
If you have to click on a furthur page to get to the shopping basket then it is ok.

If the only point of the page is to sell than don't link.
Whereas some websites that do offer valuable info on a subject with an option to buy as a secondary thing (such as groups' sites, authors/book review sites etc) I think it might just be ok to link to it (judgement call on that one).

Don't know if that helps or not

I forgot to add, you might get an official Italic response quicker if you post on <./>feedback</.>

As Mort says, it's all a manner of tone and intent. If the page is pretty much telling you how to buy a product, is promoting a product for the purposes of convincing you to spend money or is in fact an online shop, we'd say no. If the page is informative and pertinent to the thing you're linking from, but has a link at the side to say 'buy this item here', it may be acceptable.

If you simply want to show someone an item, it might be wothrwhile spending time looking for a non-commercial page that shows the same item. Many online shops allow you to link directly to a picture of items rather than the sale page