The least risk is?

Are there any inside tips and tricks? Do I open an account to receive a paypal payment, clear it, withdraw the funds and close the account to ensure that it cannot be reversed? Do I do this for each transaction? Will this be difficult with first time seller rules??.

I think alot of people have their PayPal account connect to separate bank account which they the periodically empty to their current account. I not sure that your PayPal account can be overdrawn. Hopefully someone can give more info on this.

As a completely separate question: are there huge duties and clearance fees that will have to be paid when these frames reach British shores?.

Yes.
Personally I'd be up front about posting from SA, and that all duties are the responability of the buyer. As someone based in Norway, I never expect the sellers of the bits I buy to be experts on the various duties involved in importing goods here. That said I have asked that certian values be marked down to reduce these costs, but only at my own risk. I have heard of the buyer protection rules in the PayPal system being used to abuse this.