I could just say

"Hey guys I have put the belts on sale" and add a picture.

It's the way I say it. The tone. Should it be professional or casual.

Maybe both. Maybe that's the solution. I both post this professional looking graphic and write "hey guys, I've put the belts up for sale." 

 I think you got it man 
throw out the casual post first and then the infographic. or the other way around. maybe just not at the same time