i usually email the seller. and give him a few days. (be polite. i think ebay monitor your mail dealings. in case of disputes. they can see how you acted with the seller etc.) then after the usual no response i open a dispute. in which they have time to answer. which they usually ignore and paypal usually find in your favour. and give you your money back, it includes postage too. there is no worries on bad feedback as you can't leave feedback when paypal get involved.
one lesson in disputes though.
i bought an ms os on ebay for £40 from ireland using a cheque. i got some crud appology from the guy (after he had cashed my cheque.) saying he had been let down and been sent pirate copies. and was thousands of pounds down the drain and he couldn't give my money back. i filled out all the forms for ebay (not paypal.) and got a refund. but there was a £15 fee for admin? that was the only time i felt let down in a dispute... but it taught me a lesson! i always stick to paypal they don't do admin fees.
there are some sob scammers out there. i've had a lot of ebay dealings and so have had a lot of people trying it on. don't be shy about your rights. it takes some arrogance to sell rip off gear. put them in their place. :-)
