1) Yes, you can buy from KR stores and use it in your MK, if it is an item for MK, an example is rings that are in common use of the races, now stones that give skills, this is on the MK or KR screen.

2) The MK items you get on the black market on map 5, you have a very low chance of dropping such an item, it is best to expect to have an item A event on the black market and when using a lucky Jera set. If I'm not mistaken, only 4 item A events are held throughout the year.

3) Potor items provide the bonus and skills if you use all 4 parts at the same time, you will get a bonus of 6 books to use more if you want. Additional points like +7 CON on the armor are added to your current status when the full set is used.

ex: You have armor +4 potor, and your CON status is 200, when you use the full set, your CON status goes to 204 CON, if you are using a book that needs 204 CON, you will not be able to remove the set while not deleting this book, think carefully before using the 6 book bonus. Finish your set first or use this bonus on intermediate books that have no importance to your gameplay and can be deleted at any time. To delete an already used book you need a Stone Of Oblivion (SOO) that costs an average of 15m.

Wow Thank you for your detailed responses. I do really appreciate it!!!

It is important to have lucky Jera set to obtain potor.
if have all potor set user can obtain set bonus.