The usual guide for selling secondhand is 60-70% of the retail price, depending on the condition.
Magazines:
Drum mag: £63.00, so £38-£45
300 round version: About £35, so £21-£25 each
Clear 170 round version: About £35, so £21-£25 each
Black 60 round version: About £18, so £10-£13 each
Short transparent version: About £18, so £10-£13 each
Battery extension: £23, so £14-£16
As for the gun, they retail at about £249; as it is non-functional, £100-£120? It really depends on what someone is willing to pay, so be prepared to negotiate.
You might do better to split it, selling the mags separately from the gun.