There are many such websites where you can evaluate your car based on assumptions.
I'd suggest to go through 4-5 of them and take an average to start the negotiations.
If you want to bypass all hassles, simply take your car to some reputed sellers in market who deal in second hand cars. They will obviously evaluate your car to a much lower price but then you'll be saving on your time.
Its advisable to take your car personally to used car showrooms in your area and seek the best possible price.
Last year when i sold my car i did the same and with little tricks got 15% better price.
Online you can put it on OLX,Carwale or similar sites and evaluate the value.