Hello again
Firstly, like I mentioned, you should ask in the help, all or the other chat(where you can speak Spanish, for example) for a price check for the item.
After you get a price check, go over to the trade chat and type "wtb reaper outfit" with the price you get from the price check. Someone who has it and wants to sell will message you and you can agree upon a location and a price with them and trade. WTB here means Want To Buy and WTS means Want To Sell and yes, you can buy it with the in-game currency, Pokedollars and coin shop coins.
Trading in the game works when you and the person you want to trade with, both having the Rainbow badge, are in the same Pokecenter together. You have to right click on them and click trade(on PC/Mac/Linux) or just click on them and click trade (on Android), if they send the request first you'll get a prompt whether or not you want to trade, if you do, click Yes, if not then No. After you click Yes a menu will appear where you can drag pokemon from your party or tradable items from you bag, and/or enter pokedollars in the middle bottom area and click Set(in this case, the pokedollars after price check) to trade with the other person who will also trade you something, in this case the reaper outfit. When all in agreement, click Accept once and then again to confirm, click Accept again.
You can get more info on the Trade Terminology in the Trade Rules. You can ask a friend of yours for translations or use a translation website like DeepL, if you need.
I hope this helps. Feel free to let me know about any more questions you have regarding this.
Have a great day,
Fus1onflare