Is it an online ad that you could post a link? Bottom line is this Tahoe came with a 5.3. No Tahoe came with a 6.0 of any variety. If they say this Tahoe has a 6.0 (Vortec Max or not) and that it's the stock motor, they're lying/mistaken/"mis-speaking"/etc. It's either been swapped or just an engine cover or door badges that someone put on it. If it' a swap and it looks factory-original (which is easy to do since it's practically a direct bolt-in) and it as well as the rest of the Tahoe checks out ok, maybe you could use the swap as leverage for a better deal, citing that the engine is not original and has unknown mileage and condition.