A Website programmed with PHP using MySQL:
Where people can register for free using their phone numbers and get their account verified via sms then they can post stuff for sale on the categories they choose. Other people can browse the categories or search. Buyers can see the seller's phone number. Sellers can set their products as SOLD/Modify.
iOS App: Same the above and linked to the same Database.
Android App: Same the above and linked to the same Database.
I have more details which i will talk about with the Programmer.