Sharing my experience:
I have sold a lot of things through both these websites especially since I was moving from Bangalore to another city. I sold my bed, mattress, study table, tv table, phone, couch, misc electronics etc.
In my opinion OLX > Quikr.
Tips : 1) Instead of giving your phone number in the ad, give your disposable email address. Believe me if a person is interested, they will drop an email and you can always reply from your phone and then eventually take their number and call them. I have done all deals in this manner as I'm very picky with my cellphone number and spam coming on it through calls/text etc.
2) Talk to the person and meet them somewhere in public if possible especially if you will be selling your cellphone or some other portable device. Don't invite them home if not required.
3) Take good and multiple pics of the item that you want to sell, believe me this will put you ahead of everyone and will gather more interest from the person who wants to buy.
4) Please be patient and smart. There will be low balling so learn how to deal with it smartly and calmly. Multiple occasions where people have low balled but i remained firm and then these people came back and bought it from me. It will happen so relax.
5) I would not encourage any woman to deal through these websites alone. Please have someone with you and follow point 2 strictly.
These services are crap if you live in small city but awesome if you are in a bigger city. Quikr is crap to be honest and they spam quite a lot in the form of unsolicited emails. Olx is a much cleaner and better service imo.