Fixed price, buy it now listings can have a 'or best offer' function enabled, but it is up to each seller if they are prepared to accept offers below the asking price and use the feature (if the 'Make an offer' function is not available, then odds are the seller won't accept one).
You can refine an individual seller's items a few different ways.
If they have a store, click on the red door icon and you can refine their listings by any personalised store categories the seller has set up.
You can also click to see seller's other items and use the menu on the left-hand side to refine their items by eBay's categories.
Or, you can refine by using a keyword search, either in the store format or general 'see seller's other items' result.
