Seeing as the seller has responded to my "£100 special" remark with a laughing smiley, I'll back myself up.
Let's start with the fact that the gun is £125 brand new, as mentioned above.
The gun in the advert is absolutely
filthy, so we can only assume has been neglected with very limited maintenance. With the amount of dirt around the trigger, dust cover and magwell, it's safe to assume it has some amount of detritus in the gearbox and/or motor.
The exceptionally rusty forward assist, while not functional shows its been used in the wet (not a problem) and then left to fester, not dismantled and dried between games.
A handful of cheap accessories a sling and some mags do not justify the additional cost, these things are very cheap and hold very little 2nd hand value.
Also, there's no photo of the T1 red dot. Get one of those added as soon as possible or the advert could be removed.
And finally, the FPS is incredibly low compared to normal G&Gs.
Your 250 on a .25 equates to around 280 on a .2. If someone wanted to buy it, a spring change would have to be a necessity.
Overall, a £100 project gun. At best.
I'd recommend pitching it for £100 without the extras, and sell them separately for a reasonable 2nd hand price.