Those barcodes are ISBN-numbers, which are used to identify your books/comics. Where I am (Sweden), they are free to acquire, and the print-shop is required by law to send a certain number of copies to various libraries, to get it all registered properly - but things might be different in the UK. I recommend researching this - though for a small, self-published print-run, you don't have to have ISBNs, at least here.
Again, seller's permits and such is something that varies by region/country. Here, I don't have to have one if I only table once or twice a year; if I do it a more regular basis, I have to.
I would say that you should have a clearly visible NSFW-label on comics that have such content - some cons require it, but even for the ones that don't, it's just good manners to do it. A lot of visitors to comic cons are minors and children, and they - and their parents - should have fair warning before they pick up an NSFW comic and start flipping through it.