> I'm thinking of building an e-commerce site in php.
>
> Anyone got any advice in building one?
>
> What is the best way to implement a payment system?
>
> Are any legal issues involved?

the main legality you will come across is the responsibility to ensure that
users data is secure.

Is there a reason you wish to build 1 from scratch? it is not so,ething you
should take on lightly especially for 1 person doing all of the coding. A
better idea would be to find a good open source ecommerce package and build
upon that.