If you are interested in purchasing a piece of work you can do one of the following:

Contact the artist by phone or email or via the Contact me page on their Pop-up Gallery

Note: This is advisable for large pieces

If the seller has added a PayPal button to their listing simply click through and make the payment

Note: If P&P is not included on the listing contact the seller to find out the cost

PayPal is the easiest and safest way to pay, see here about setting up an account
If the seller has selected Cheque please follow the artist’s instructions
Contact the seller to arrange a Bank Transfer or pay by credit card if that option is available

Note: You should expect a delay before the seller sends out their item to allow payment to clear