if you like to sell stuff

[PowerDiver]

The current silly system where the shopkeeper knows what you know can be turned to your advantage. If you find a ring of reckless attacks, put it on for one quick kill to learn the plusses. Be sure not to get attacked while wearing it, and you can sell it to the anguished cries of the shop keeper who for some reason does not expect the AC penalty.

[PowerDiver]

There seems to be a bug in that you do not learn attack plusses on barehand attacks [even though the bonuses do count], so be sure to wield a weapon when you use this trick.

[dos350]

no weapon attack is a hack job and this is leet, ty. please no rage~

[Raxmei]

The current silly system where the shopkeeper knows what you know can be turned to your advantage. If you find a ring of reckless attacks, put it on for one quick kill to learn the plusses. Be sure not to get attacked while wearing it, and you can sell it to the anguished cries of the shop keeper who for some reason does not expect the AC penalty.

Back when I sold I threw them to get the combat pluses at some risk of breakage. If I still did it now I'd just save them for town and wield them while I go to town on some defenseless townsfolk who don't have any attacks. Also try putting on rings of the mouse and immediately taking them off. A nice +dex +stealth ring turns into garbage the moment you attack while wearing it. Them and the remarkably costly rings to teleport and escaping used to give me a fair bit of my early money.

[SSK]

...and the remarkably costly rings to teleport and escaping used to give me a fair bit of my early money.

(By the way I don't sell anymore; no_selling is a far far better game)
I've discovered a very important use for Teleport and Escaping rings:

I am trying for fewer-turn wins. It is rare to find speed items early in the game, and I find the early game consumes massive amounts of game turns--but if you have regeneration and a little speed early like from these rings, I find you burn much fewer turns enabling much quicker wins. Wear one of each and you get 16 energy/game turn vs 10 = 40% less turn usage in the early going.

I am about to complete my first dive win: I am ready to take on Morgy at about 550K turns, and I think my wearing these rings, plus a much faster dive to about DLevel 31 are responsible.