While such a filter would be useful, there may be some techniques that can be used with the current search to eliminate some of those unwanted results, albeit with some effort.
eBay does use some "smart" programming to substitute keywords, but those features only work when not using features such as excluded keywords, quotes or parentheses to perform Boolean "OR" searches. And eBay currently lacks a feature to exclude a single category while searching other categories.
Excluding postcards from your results may be as simple as using an exclusion term that appears in the title or item-specifics of the items you want to exclude, such as -postcard. Additional listings can be removed from the results by using other exclusions.
If you have some examples of searches you want to perform, and sorts of items you want to exclude, someone may be able to provide some specific example search links to provide a starting point for future searches.