Unlike e-bay there is no invoice generating feature on Jackpot.
Find out how the seller will accept payment and forward it to them that way.
If they accept paypal and tyhat is how you want to pay, get the sellers paypal I.D. and go to paypal.com and just use send money feature.
You must be registered with paypal to do that but it is free and simple. If the seller doesn't take paypal then they need to send you their address so you can get them their money.
Hope this helps.