Phone stolen! I think i know who took it.?

[Tessa]

I lost my phone recently in school and someone took it. i saw an add up on my school's black market facebook page. A guy was selling a phone with the exact same specs as my old one and he goes to my school. I screen shotted the evidence of this post and a day later he took the ad down... (idk how he knew, but its pretty stupid to put an ad up for the phone u stole from school on the school's bm page -.-) so now what can i do? I am planning to contact the police but what can they do for me? will i ever get my phone back ?