eBay cross promotion rules are one of those features which it is easy to ignore. These rules define the items which are shown in the cross promotional box underneath each listing, showing other products which the seller has for sale.
eBay does not make it obvious how to change these rules and I suspect that most sellers probably just leave the setting as default, where eBay selects the items which are shown in the box. Rules can be changed in the marketing Tools section of My eBay. I have just changed the rule to show items from the same shop category, although items can also be shown according to other rules such as a keyword search.
What I found really annoying about setting these rules is that eBay give absolutely no indication of how effective any change will be. The limited traffic stats available to eBay sellers only show page views and not where the traffic has come from. This means that any change in cross promotional rules is a stab in the dark. I think that it is eBay’s interest to provide better statistics as better informed sellers (should) means more successful sellers.