]‘With a sense of what sells rather than what’s cool they could well clean up.’

The NME, belatedly picking up on the AC/DC story, sent the veteran Phil McNeil to interview them. Barely stifling a yawn he allowed that ‘With a sense of what sells rather than what’s cool they could well clean up.’ Again it was left to Sounds to make the case for the defence. ‘The rhythms hit your heart like a trip-hammer,’ wrote Phil Sutcliffe, waxing lyrical about the ‘fluent solos from Angus’ and ‘the fascinating stage presence of Bon, leathery debauchee, a strange companion for the school kid’.
1,What does "With a sense of what sells rather than what’s cool they could well clean up" mean in other words?
2. What does " allowed" mean here?
3. Does barely stifling a yawn mean he was tired.