I register domains that could represent great upgrades (keyword or brand-type) for a large amount of potential buyers. There is NOTHING unethical, unless I am targeting specifically ™ holders - which I am not.
I also never put ADS on my domains.
I do not actively contact companies trying to sell domains
IF, out of my +40,000 domains, I get 1-2 emails / letters from lawyers that prove they they have 100% rights to the exact term in my domain, then I happily give the domain to them for free (this can happen when you have large amounts of domains) - but this is very rare as I strive to go for domains that have nice-looking keywords, like the examples below...
These types of "brand" domains sell every week; a few examples of these types of domains that I try and buy (brand-centric)
artline.de - $ 6,612
MediNow.de - $ 2684
cg-group.de - $ 3,331
Tailwind.de - $ 6,628
webradio.at - $ 4,697
cortexgroup.com - $ 4,188
RDTGroup.com - $ 1,800
netid.at - $ 9,263
myplace.at - $ 10,640
tiptop.ch - $ 10,835
m-plus.ch - $ 3,318
transport24.ch - $ 3,700
If you own enough of these types of "brand" names, they will sell almost daily.