Unless it's an unbelievable offer (£40m or more), I wouldn't even consider selling Mane. We wouldn't be able to replace him over the course of transfer window, in my view (and we'd be potentially screwed if we didn't replace him).
But Wanyama, it's a totally different story. For me he's been poor for a while now - the Arsenal game aside, when he was right back on it (I wonder why?) - and today's red card just had the feeling of last straw. He was truly awful in the home game against Spurs, for example, in my view.
If someone wants to offer us £20m for him, then absolutely accept. He'd almost certainly be off in the summer any way, due to only having 12 months left on his contract. So what's five months earlier? We wouldn't even necessarily need to use any of the funds raised to directly replace him. Romeu and Reed could easily be given more game time between them, between now and May, as makeshift cover. If that £20m gets us a decent striker and winger, then it would be a massive thumbs up from me.