If the paint or decals on one side are different to the other then obviously something has been changed. What you need to find out is why.
Best thing to do is ask them why it is in that condition. By law, if they know the real reason, then they have to tell you. If they are reputable, and know the facts then they should not mislead you. You would be able to pull them up on that at a later date if you found out anything else.
There are many reasons why it could be like that. 125's are generally a first bike, and therefore mistakes can be made whilst riding or manouvering. It is possible it could have been dropped at a standstill and only the part in question was damaged, then replaced. But also, yes, it could have been crashed, so be wary and look for other damage.
Question is, if the bike has obviously had some sort of damage in the past, is it reflected in the price you are paying for it? If not, then why worry yourself about what has happened to it. Find another one, move on and leave it there.
I had something like this done to me on a £6,500 GSXR750.... Its not nice when the doubts start to creep in.