One plausible idea was cheese. The French love cheese (as Nick pointed out, they have loads of different varieties) and could have been curious about a good British cheese. The French chef on the You're Fired show suggested Stilton. Paul went for Cheddar instead, apparently on the grounds that it sounded very English. He was impressed by the quality of the Makro product, as were the others when they tasted it on the ferry. With hindsight it now looks like a dumb idea, but it was almost right, and who knows, if Adam had taken the trouble to cut it into smaller pieces and presented it well it might have sold.
The sausages were a good product. Getting equipment to cook them with was also a good (even necessary) idea. He says he couldn't find anything suitable - I'm guessing that there were better gas burners in Makro but they were too expensive. He said he didn't have time to go anywhere else. Or rather, not to any of the places on the list they'd been given - they couldn't just pop into any shop they felt like. So he improvised. He got one of the jelly burners, and had the wit to know he'd need a wind shield so he got a can to use for that. He even knew the wind shield would need holes for ventilation. The main thing he missed, I think, is that the pan needed a lid to stop the heat escaping, and maybe he needed more than one burner. So basically he was stuck between a rock and a hard place, had a good idea to get himself out of it, and it didn't quite work in practice. It was enough to get the sausages brown and delicious-looking, but not to cook them. So close.
Arguably he was struggling with the rest of his team. Kristina showed she could get the sausages cooked, but she took her own sweet time over it. Adam just dumbly persisted in trying to get the burner to work, never succeeding, and I think he wasted a huge amount of time on it (much longer than Paul). Ghazal does nothing the whole show that we see. Someone could have cut the cheese up, and arguably this falls under Adam's purview because he was in charge of marketing and making the stall look good (hence the sign, which he also got wrong). But that would involve showing some initiative which is not Adam's strong suit. Kristina has plenty of initiative, but unlike with the sausages she has no vested interest in sorting out the cheese because she's disapproved of it from the start. Ghazal does nothing.
Katie might have done something but Paul thinks he needs her selling to shops. Which again is a plausible idea since a shop can make far bigger orders, but it doesn't quite work because it takes the French speakers away from the stall. Apparently Kristina was assigned to their team as a French speaker, because she has a French qualification, but (through no fault of her own) she was useless at it. Maybe it would have been better to send Katie with Adam while Paul remained at the stall - but this is hindsight again.
Selling the sausages at cost price was plausible. At that point it looked like they had no way to cook them and were going to be left with a lot of expensive stock at the end of the day - as happened with the cheese, which had to be dumped. However, by then Kristina had actually got them cooked (without telling Paul of this) so Paul looked like an idiot.
That's all I can remember off-hand. It's worth remembering there was a kind of group-think going on. Everyone thought he was doing well, right up until they were told they'd lost. Except Kristina, whom he thought was merely game-playing and making criticisms in order to prepare a boardroom defence. Which again was plausible, but wrong. If one other person on his team had spoken up he might have taken her more seriously, but they didn't. His mistakes were really mistakes of the whole team (except Kristina).