Grilli alone isn't going to bring back a prospect good enough to make it worth declaring yourself "sellers" if you still have even an outside chance at the postseason. Grilli is not getting Pentecost from the Jays. He is not getting Guerrero from LA. He is not getting Baez from the Cubs. He is not getting anyone that could realistically contribute to a championship caliber team, so I am against trading him away until the season is very clearly lost...and it isn't...yet.
However, Grilli might be good enough to bring back a talent like Pentecost or Guerrero or Baez if he is paired with some other pieces like AJ, JJ, Uribe and KJ. He might be good enough to make a team take on CJ's contract. I would be all for moves like those once the playoff hopes are clearly dashed, but I would never be OK with trading Grilli (who can contribute in 2016) for some C prospect that will barely make the Braves top 25 prospects list, no matter how bad the team is this year.
Not sure if you're referring to Austin Barnes as that prospect, but if so you might consider that he's the #9 Catcher prospect according to Callis and Mayo (MLB Pipeline) and is hitting .300/.381/.483/.864 with 8 HRs, 11 2Bs, 1 3B, and 9 SBs (with a 26/24 BB/K Ratio) in AA. His profile with the bat is almost exactly what the organization prizes with the new Seitzer philosophy in place. Their scouting report on Barnes states...
"Barnes focuses on making contact and using the middle of the field, an approach that has helped him hit for average throughout the Minors. He walked more than he struck out in his first four pro seasons and projects as a high on-base percentage guy with gap power. He runs better than most catchers and has good instincts on the bases."