Find an online shop for what you want/need. Try
http://www.freefonefun.co.uk/ and work from there. Establish that the
product does do what you want it to.

Go back to eBay UK (www.ebay.co.uk) and try to find the same product.

Check the sellers feedback ... personally I have had no problems on eBay
(touch wood) but many would suggest that you go for someone that has at
least a few comments of feedback ... which must be favourable.

Use eBay's option to ask any questions you have. I usually ask any old
thing, just to see how long they take to reply and in what manner that they
reply.

That's it, make a bid and keep your fingers crossed.