numb3rc said:
I'd agree with macd21. Even the most ignorant shopkeeper would probably notice that Eldar weaponry are not made of metal, plastic, or in fact any identifiable substance and look inhuman. They'd be suspicious. However , give your techpriest some plating and some time and he could probably do a decent job in disguising it as an exotic needler-variant. It wouldn't stand up to any close scrutiny but it might give you the deniability you need for a moment.
Of course, the high value of the catapults is down to their rarity... basically, the PCs will want to sell them as Xenos weaponry in order to get the best price for them. If they try to pretend that the weapons are human manufactured needle weapons that use a type of exotic ammo that no one can manufacture then they won't get nearly as much for it. Much of the potential value lies in their status as exotic xeno-guns.