1- Yes you are allowed to open a screenie shop, in fact there are few of them around here
2-Prices I've seen regarding screenies often range from 50cr to 200cr, I suggest you have a look around some screenie shops in the premade section, for some notion of what to settle your prices for I guess
3-You can always put a watermark on your screenies, and when you recieve the payment, you can send them to the person who bought it
4- Yes you are allowed to open a screenie shop as referred before, you have 3 options
If you want to make your own screenies and then sell them open a shop in the premade section.
If you want to make screenies according to other ppl's ideas, and stuff, open a custom shop
If you do not wish to charge for your services you can open a free shop