Timlin-Guy

You can have components just they have to be screen individually. Though I've never brought something like a PSU only HDDs

Customs will likely make you pay for the stuff you buy, unless it's personal items (clothes for example), I don't know how the uk process works but generally you have to tell them what you brought back and they can search you stuff to check if they want to.
 
Also I'm so glad I don't have to pay sales taxes NH ftw

also in usa they would think the pc parts are bomb parts ... you know how stupid they are these days lol 

This idea seems like it would be successful  , if you do it right. You have to make sure there is enough of a market for custom pcs where you plan on setting up shop and also remember that once a customer buys a computer from you he won't come back for a couple of years (unless they are a extra enthusiast but if they are they would probably want to build it themselves )  to get customers to come back you could also provide repair services.
 
For building Pc here are some thing you should provide:
 
Things that are hard with out proper equipment and training like Hardline water cooling or painting computer parts
 
Being able to design any Aesthetic a customer might want 
 
Sell more then just pc get peripherals to 
 
Professional Pc advice 
 
Decorate your shop with a 'gaming theme' (leds, cool logos, computer cases all around)
 
Good Costomer service
 
 
 
 
 

It's an incredibly niche thing. Someone who doesn't care about high performance PCs will buy a prebuilt, someone who does care about high performance PCs will probably build it themselves. It would be hard to make a profit off of custom system building alone.

Maybe...
(Sometimes I wonder why I even live)

my latest build: Mjolnir   I really love all mat black with gold   Parvum Ssytem M1.0 whit triple layer option Intel i7-5820k  @ 4.2 Asus Sabertooth X99 16gb (4x4) Gskill DDR4 2400 2 x GTX980 reference SSD Kingston Hyper X 120gb 3 x Seagate 500gb 7200rpm 2.5" HDD EVGA 1050g PSU  
 
 
 
 
 
 
 
 

Fuck me this sounds shady as fuck, and quite rightly illegal. 
Government deems it an illegal and invasive practice, unless they want to use it as a method to charge the general public. Cunts.