Ebay has saved searches that will alert you when something matching it is posted.
Yeah, but those alerts are only sent out daily, not moments after something is listed. eBay somewhat randomizes the time of day they're sent out, so as not to favor certain recipients.
There are third-party scanners that will show you new eBay listings whose titles have specific keywords/phrases. I used to use one that ran in the background and popped up for a new listing that matched. Haven't used it in a while, though. You'll end up looking at a lot of stuff you don't want in order to get one that you do
Was it a sister site of "the forklift"? I think it was called "The Hydra". That was killed a few years ago. I looked at doing this myself a while ago but it's against eBay's TOS to create bots to purchase listings, so you'd still need to sit in front of a computer and monitor the listings.
Their API does have some useful features for making a scraper, but the request limits are too low and the results are inconsistent. Depending on the server, you will get different results for each search, some new items will show up and then disappear, others won't appear at all until 5-10 minutes after listing.