As someone who works with teenagers I would say go for it. You clearly have the drive and ambition to attempt something and are not too bothered about it potentially being a waste of time or not making a worthwhile profit. Its great to see a 15 year old be bothered to try to do something!
However, yes as pointed out by many there are lots of pitfalls and by starting a business there are things which will need to be in place which will cost more than the £20 start up price you quoted and it will be hard work and potentially time consuming but do not let that put you off, nothing ventured nothing gained!!
Its doubtful you have thought enough through to build your own brand as it were but every great businessman started somewhere, Who knows maybe a store or brand will end up buying you out.
I doubt you would make good money from it, well enough that it is worthwhile keeping on top of everything but I would be more than happy to have been proven wrong!!