Welcome to Void! *Throws confetti* Make yourself at home on game/forum
To answer your questions:
1. P/C means Price Check, like others B/O is Buying Offer and C/O is Current Offer
2. Laby doesn't have Black Mesa, only Arch Devil (AD) and Celestial Master (CM), but soon to come!
3. 5/5 is for Head/Hat with top, glove, bottom and shoe or accessories (depending on the count with the weapon itself 4/5 is same concept but a set with one piece that is exchanged with another. I.e (4/5 LoW (Lord of other World) with skill attire hair))that is one example though it could vary but when you hear 5/5 or 4/5 its assumed costume pieces but if its accessories of the costume it would say combo 4/4 or combo 5/5 etc
Hope this helps and feel free to ask more questions