I've seen those ads on YouTube too claiming they'll pay like 10% over market rate or whatever.  Definitely makes you go hmmm. 

I mean if it sounds too good to be true, it always is right? My guess would be either straight up scam trying to lure folks in or just super low liquidity so good luck trying to actually sell at that inflated rate they claim.  Either way, probably best to just steer clear and 

stick with the reputable mainstream exchanges that have been around a while.  May not be quite as flashy but least you know they're legit and you can cash out your coins if you need to.  Gets too dicey playing around with these fly-by-night operations just trying to make a quick buck.  Better safe than sorry when your hard-earned moneys on the line.