A 960 rated player is not a pro. If someone's at that level and aspiring to get better, then I'd definitely recommend they play up for minis.
The only way I'd call a 960 rated player that plays advanced a sandbagger is if they intentionally threw poor tournament rounds to keep their player rating lower than it actually should be. If someone really had 980 skills, but bagged during tournament rounds in order to have a 960 rating and appear to be an advanced player in order to play down in minis and will plastic to sell for a profit I'd not only call them a sandbagger, but also a complete [I'm a potty-mouth!].