Can you tell how easy or difficult "sells jobs" keyword to be optimized?

I checked in Google, first coming webistes are around 10 years old then how to overcome domain age?

Also tell me others thing about the competition that the competitor can be defeated.

Tell me if I am not able to go for paid backlinks then how much time I have to give for it.

 

It would be very difficult and not worth it.
The page rank for the first 4 are 6, 6 , 5 and 5 meaning they are very strong.

If a website has a page rank of 5 or higher the competition is too strong and isn't worth it.
If it has a has a page rank of 4 but the keyword is in the title leave it alone if it it's 4 or higher without the keyword in it's title go for it

1-3 are easy to rank for and the competition isn't strong

 

Apart from pagerank of your competitors and domain age, This keyword is very competitive and its very difficult to see your website in top positions of search engines. It may takes several times . So you can choose another keyword.

 

It would take a lot of effort to overtake aged, PR5 / PR6 sites.

 

You do mean sales jobs, right? In which case, it is quite competitive and you'll have a tough time breaking into the top 10 without a good few quality links.