Is it fair for seller to pay buyer $6k in closing cost?

[Tom Shi]

I am selling my house in Portland Oregon. Housing inventory in the city is scarce, so still a seller's market.   Asking price was $410,000 and a few days after listing I received 1 offer, at asking price.  So far so good. 

After inspection, it was found that sewer is in need of major repair (bellies in the line). The sewer inspector labelled a failing system.  Estimated cost to repair is around $10k. 

There are also lot of minor things on the inspection report such as missing paint, a leaking faucet, minor concrete cracks, old vermin droppings in attic, minor mildew, etc.  It's a older house, so no surprise there a lot of stuff going on.  

The buyer is asking me to cover $6k of their closing cost, in addition to requesting me to fix the the sewer line.  
I am willing to pay to fix the ailing sewer.  But the $6k they are asking me takes me aback.  There is no way the total cost of fixing all the minor stuff mentioned on the inspection report even approach that amount.  My realtor doesn't see it as outrageous demand and seems to subtly suggesting me to accept this deal.

So my question for you guys is that does is it common to have buyer ask for so much in closing cost credit from seller?  Am I just out of line to think $6k is a crazy amount to ask the seller?