I've not checked any of these for sites/domain availability/price but I'd be going through ones like these (if appropriate)
buy watches
vintage watches
sell watches
watches online
watch store
watch shop
watch dealer
watch seller
watch broker
and so on. I know you jokingly mentioned Acorn Watches, but I'd honestly rather build on AcornWatches.co.uk for reg fee than watch trader at £5k.
If you go too generic and more than 1 word, you end up losing traffic to people messing up navigating to your site. You need to more than compensate for this by direct search engine traffic benefit, or its simply not worth it. Given that "watch trader" is going to provide no visitors, I think you lose out here.
In the discount voucher niche, I can see lots of traffic landing on my site when they were looking for a different one to begin with - presumably they just couldn't remember the generic phrase of the site they were looking for, so start typing domains into google.
I'm undoubtedly losing the same traffic from people looking to find my site, but I more than make that lost income back because "discount vouchers" as a search term is sending more than twice the traffic of the closest keyword. "watch trader" is going to lose you more traffic than it gains you, imo.