I'd say ... Sell the 400D. The lenses, you can wait a bit and see if you need them on the 7D.
BTW, there is no such thing as a loss selling gear you never use. You can always use the proceeds to get stuff you really need.
Actually, I wouldn't much miss the 18-55. The 55-250 I'm not so sure about though - it is very compact and the image quality isn't bad at all. The 18-55 I could sell without the filter and put the latter on my 70-210.
The thing is I kinda like the 400D. Looking at the back it's like a mini 5D MkII with the buttons on the left, one of the reasons I bought it second-hand at the time. Emotions get in the way of reason...
5D3, 5D2, Sony α6000, G16 | SY14 f/2.8, Ʃ20 f/1.8, 24 f/2.8, 35 f/2, Ʃ35 f/1.4A, 50 f/1.8 I, Ʃ50 f/1.4 EX, 100L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 70-300L, 100-400L | E-mount: SY12 f/2, Ʃ19 & 30 f/2.8 EX DN, 16-70 ZA OSS, 55-210 OSS, Metabones SB | FT-QL, AE-1P | FD(n) & FL lenses