Ok, let me try set prices of some pokemons. Also want to mention in the beginning that I won't make prices for all of pokes which you have suggested Mestredo. Reason is simple, we don't have resps of all of them and set the prices for them is pointless IMO since who gonna sell you for example Meltan which was only available on summer event.
Also I would like to refresh Shadow Mist's words, which show how prices are set (beginning I've changed couse there were exactly given pokemons):
Pokemons won't get price for now, as they spawns have been added within less than 2 month, which is minimun time required for them to have NPC price.
Why I posted it here? To show that only prices can be set (if is still actual) for pokes which spawns exist minimum 2 months and at least half of your suggested pokemons don't have any simple resp. So in conlusion don't expect prices for pokes which we only could get in some events.
Ok, now let me set the prices for existing pokemons in game - some for them will give argument.
One argument for all of them - we can get them every 55/65 days (if I am not wrong) so there is a reason why NPC could buy those pokemons, couse everyone who makes daily tasks can get them so they aren't so rare as we can think (just need to start doing tasks and that's all).
* Malamar = 15k
* Cryogonal = 15k
* Skiddo = 10k
* Gogoat = 20k
* Espurr = 10k
* Meowstic = 20k
* Larvesta = 10k
* Volcarona = 20k
Also I am thinking if these prices aren't too high, becouse everyone can get them, but from the other hand haven't seen so much people are selling them for price as above, usually is 2x/3x more. It's up to you right now to change them a bit or make it more balanced.
Next ones:
* Gothielle = 10k - there are just few spawns with its single respawn (if I am not wrong again). Also it doesn't get extra chance so is not so easy to have it (don't talk here about aka lucky players).
* Kecleon = 1.5/2k - it's fine IMO, couse is possible to catch it
* Goomy = 1.5/2k - as above
* Sliggoo = 4k - can not be caught, but easy to lvling up Goomy, so
* Goodra = 6/7k - with a bit effort can be got by lvling up Sliggoo, so
* Lycanroc Forms = 1k~ - possiblity to get by lvling Rockruff to 40lvl (and it is quite easy to catch) so propably won't be abusable like it was with Dugtrio e.x. (and with price 4k as you suggested would be easy to get nice profit so fast), also making its price around 1k~ won't encourage many people to catch many Rockruffs and exping them
* Fennekin = 50hd
* Braixen = 300hd
* Delphox = 4,5k
* Chespin = 50hd
* Quilladin = 300hd
* Chesnaught = 4,5k
These are mine prices. As I said, don't know if they are correct. If no, then change more or less.
Also didn't mention about Cloneds: Primeape, Weezing, Dodrio, Victreebel, Electivire, Magmortar, Rhyperion, Ninetales, Dragonite, Scyther, Scizor; didn't mention Alolan pokemons: Sandshrew, Vulpix, Raticate, Diglett (?) Dugtrio, Gumshoos, Comfey and other one is Diancie cus simply don't know how are their prices between players.
Regards!