You can block up to 10 sellers' items from showing up in a search result using the filters in Advanced Search or in the left column of the search results page (More Refinements: Seller). You find the filter where you limit the results to those of given seller(s) and change the "Include" to "Exclude" and list up to 10 IDs in the following text box.
You can then save the search (link towards the top left of the results) and then access the saved search in My eBay: Saved Searches (you'll probably want to turn off notices of new listings while you're there) changing the keywords and other filters as needed to find what you are looking for. You can save multiple searches with up to 10 sellers excluded in each to use for searches in different categories (or other way to distinguish which set of sellers is most likely to have listings in what you are searching for).
