The reason he doesn't sell merch is cos they haven't presented him as someone who should... Harley Race never sold a T-Shirt in his life, nor did Nick Bockwinkel or Bob Backlund... In a way, WWE is trying to make Barrett a throwback, like those kind of guys and it could work... if only they'd let him loose with it.
The problem Barrett has I've mentioned in other threads... first he made a fuck up by saying he'd like to get out by 40... giving the E no real return if they push him. Now he might have been quoted out of context or have smoothed that over, but he's still gonna do his time in the shitbox like Triple H and others did...
2ndly he has had his "brush" with mortality in the ring... That elbow didn't chip... it seperated clean in 2... he's lucky to have an arm that works, much less be able to continue as good as before (which he is) but inevitably, he has a bit of a "block" on what he is confident doing in the ring... "not showing your passion" as JR puts it, could easily be..."my arm isn't right even now, I don't want to hurt people so I will take it easy..." and he'd be right to do that... but that's not what the E want in main eventers... they want Cena's, Edge's who could literally be crippled but still go 150% at a house show.
Thirdly they are not using his one marketable trait... that no UK guy has won the title... Davey Boy was close but never quite made it, Regal blew his chance... the time for Barrett was ripe right when he got hurt, had he not been he'd have had MITB and won the title from Sheamus... UK v Ireland at a Dublin or London show for a rematch or win for Barrett would have done monster business... but Ziggler fucked up and Barrett saved him, ruining his window. Now Sheamus is done as that main eventer too...
At the end of the day I would push Barrett, give him the title 3 times... once a short reign as a heel, once a big face win in the UK for a Summerslam or similar and a final one leading into him putting someone over to replace him... Davey Boy Smith Jr (Harry) would get my vote for that... Give him Regal as a manager and he'd instantly rise... partner him with Neville in a new "Bulldogs" and he will rise... put him with the Wyatts and he will rise... but Barrett's best thing is the one thing Vince seemingly ain't bothered about, the British fans would embrace Barrett if he was presented right... and that would be enough for at least one reign... but at worst case, he gets to be this generation's Scott Hall or Curt Hennig (hopefully without the booze or drugs), the best guy never to get the belt...