Depends on the Market and the motivation of the Seller.
In a hot market, you can generally "forgetabout it" unless the list price is inflated.
You can sometime make an adjustment in the sale price Upward to cover the buyers closing cost with out too much concern from the seller, if the house has no problem making the appraisal.
Generally, just asking for Seller to pay the closing cost is considered by most agents to indicate the marginal ability of the Buyer to perform to contract... it does not provide a warm feeling. Unless the market is poor... you will very often find such offers rejected with out counters being made.