Sometimes shopkeepers may try to cheat you. The probability of this mainly depends on your Charisma. It will never happen if your Charisma is 10 or more and will always happen if it is 7 or less. If your Charisma is 8 or 9, shopkeepers will sometimes try to cheat you, with a probability dependent on your Luck. On a related note, many services will not work properly if you are hallucinating or confused.
The price of services will be mentioned under the individual service headings, but in general, it is strongly dependent on the object on which the service is being performed. Your charisma also affects the price of services, but to a lesser degree than it affects the price of objects.
If you ask for the identification of an item which is already identified, the shopkeeper may try to cheat you, but you will be warned by the message ``<The shopkeeper> chuckles greedily...'' before you pay any money.
After you've paid for the identify, a few things can still go wrong. If you're hallucinating, ``You hear <the shopkeeper> tell you it's a pot of flowers'', and if you're confused ``<The shopkeeper> tells you but you forget''. If you asked the shopkeeper to identify an item which he/she doesn't sell, a basic identify has only a 1 in 2 chance of success and a premier identify a 1 in 4 chance.
If you do not know that an item is cursed (that is, you are not a Priest or Necromancer and it is not marked as cursed in your inventory), then the shopkeeper may cheat you by taking your money and not uncursing the item. You will know that this has happened because you will get the message ``<The shopkeeper> snickers and says "See, nice and uncursed!"''. The shopkeeper will never warn you if the item was already uncursed, presumably because if they did, you could determine for free which of your items were cursed.
If you're confused, ``You accidentally ask for the item to be cursed!'' and the shopkeeper curses it. If you're hallucinating, there is a 1 in 4 chance that ``Distracted by your blood-shot eyes, the shopkeeper accidentally blesses the item!'' but a 3 in 4 chance that ``You can't see straight and point to the wrong item!'' and nothing happens.
This is another service which does not work when you are hallucinating (``You hear <the shopkeeper> say it'll "knock 'em dead"'') or confused (``The numbers get all mixed up in your head.''). Otherwise, you are told the basic damage your weapon does against small and large foes. General shopkeepers have a 1/10 chance of not being able to tell you anything about the weapon, a 1/10 chance of not being sure about small foes, and a 1/10 chance of not being sure about large foes.
Erodeproofing costs more the more eroded your weapon is, with a minimum of 200 GP and a maximum of 1500 GP. If the object is erodeproof already, the shopkeeper will ``give you a suspiciously happy smile'' but will still attempt to charge you (always, regardless of your Charisma and Luck).
The cost of enchanting a weapon increases dramatically with its current enchantment, and there is no upper limit to the charge. The approximate cost, modified according to your Charisma, is 100 if the weapon is negatively enchanted, and otherwise (enchantment+1)2 * 625 GP. Artifacts cost twice as much. Shopkeepers cannot enchant weapons beyond +5.
Poisoning weapons costs 10 GP times the number of items (e.g. poisoning 50 darts would cost 500 GP).
Erodeproofing costs more the more eroded your armour is, with a minimum of 100 GP and a maximum of 1000 GP. There is no warning about armour which is already erodeproof.
Just as in the case of weapons, the cost of enchanting a piece of armour increases dramatically with its current enchantment, with no upper limit. The approximate cost is 100 GP if the armour is negatively enchanted and otherwise (enchantment+1)2 * 500 GP. Artifacts cost twice as much, and shopkeepers cannot enchant armour beyond +3.
The approximate cost of basic charging is 300 GP and the approximate cost of premier charging is 1000 GP, which is also the upper limit on the cost of charging. Wands of wishing are an exception to this rule; they cost 900 GP for basic charging and 3000 GP for premier charging.
If you're confused or hallucinating, the shopkeeper may cheat you by taking your money and not charging the item. The message associated with this is ``<The shopkeeper> says it's charged and pushes you towards the door''.
Otherwise, your item will be recharged. It has the same chance of blowing up as it would if it had been recharged with a scroll of charging. If it's a wand, and it didn't blow up, it then gets some extra charges. A wand of wishing will get one extra charge if premier charging was used, and the shopkeeper will take all your gold for this service. Any other wand will get 5-9 more charges if it had fewer than 16, or one more charge if it had 16-19.