it is preferable to exclude even positive hits, on the overriding principle that we don't want to do business with any seller who engages in spamming, even if that seller happens to be selling what we want.
Agreed. A spam blocker that blocks all spam will block spam from someone selling something you may actually want to buy -- because it is spam. If you want to go through all the spam on the off chance that something you want might be among all the spam ("There's gotta be a pony in here somewhere!"), you always have the option of turning off the spam blocker or not blocking spam at all.
I often find it preferable, when searching for something that is hard to find, to search in more than one way.
First, a very targeted search that is unlikely to return any false positives to quickly see if there is something new since the last time I searched. This can actually be automated and can generate a message when something is found -- if the item is rare enough or the search targeted enough to not be overwhelming.
If nothing turns up in that first search, then that can be followed with a search that casts a wider net -- one that does not block quite as much spam, and that is not limited to as narrow a category as the first search. The second search may return some items that the first search missed, but may also turn up items that meet the looser search criteria that are not actually of interest, and may require more time to weed through the results. This wider search can be done less often than the first search, but is often much more time consuming. The wider search may turn up items where the seller has listed in the wrong category, or has left out crucial keywords or item-specifics; items that are harder-to-find.
Another option is to browse an entire category for newly listed items, or to browse the listings of sellers selling similar items. This works best if one has a large list of similar kinds of items of interest, rather than just one specific item being sought.
It can be easy to weed out false positives. It can be easy to prevent false negatives from being weeded out. It is often difficult to do both effectively at the same time, however.
