They can ask for concessions but it will make a difference on what they offer on the property. In other words, if the buyer asks for $5000 in closing costs on a $200,000 house it is like offering $195,000. If they offer $180,000 and ask for $5000 then it's like offering $175,000. If they offer $205,000 and ask for $5000 I bet the sellers say yes.
It's all about the bottom line.