There are at least two types of buyer and their different preferences will give a big gulf in pricing.
You'll have collectors or enthusiasts of the FAMAS with no interest in fielding it so they won't try haggling down over things like replacing the hop rubber or servicing the gearbox or where it's been stored. At the same time, some collectors will see a loss of value without the original box.
On the other hand, you've got those who'd be keen to field this and they may try to haggle down on an unknown quantity, especially as it doesn't have a retail warranty. The gun may be unused but being unused gives you no proof it cycles, its hop rubber is still intact or that the gear grease is in a usable state after 30 years.
There's another TM FAMAS SV listed here for £160 with the box, a second mag and shipping included so you're being undercut by quite a margin. Your listing would probably have to be dropped by £30-40 to be more competitive, especially without the convenience of a box to ship it in.