some helps here:

im planning to get a second hand Cello instead of getting a new one
what do i need to look into?
lets say the Cello has been used for 7years
and yes, is a beginner's Cello

Go and play it and if you fall in love with the sound and how it plays and the price is right, then go for it. It all depends how it feels and sounds in your hands. If you like it, whose to say any different.

should i ask the seller to play a song?
or i learn a chord now? xD

if i feel the tone is right, that will be the one?

Playing it is good advice assuming you can play cello, and it begins to appear that you're a novice.

I've witnessed a demonstration in a music store in which the salesman pretty brashly altered his playing to push a particular piano, so having the seller perform may be a bit risky--playing a cello loudly with lots of vibrato, for example, can trick the gullible into thinking the instrument has a better tone than it has really.

Do you have a friend who plays the cello? If so, see if you can bring him round--and have him audition other cellos for you as well.

Also: look over the cello to see if it has any cracks, and try the tuning pegs.