Can anybody advertise on MedanAja.com?

No    Not everybody.    But anybody operating from the Medan-Area may do so.
The whole purpose of MedanAja.com is to be a platform promoting products from Medan-Area sellers.
So only sellers who reside in the Medan-Area or operate from a premisses in the Medan-Area are expected to sell on the MedanAja.com platform.
A good rule of thumb, for most products, will be “Can the item be delivered to the buyer within 1-2 days”.
Of course that may not be possible for some items but as long as you are local then it is OK.
There is an “Abuse” link on each sellers profile so sellers may be reported and removed if found to be abusing the platform.