I am new to dslr photography and I have just purchased a canon eos 500d kit with a 18-55mm IS lens, I have been looking at all the different types of filters and have been advised to get a uv filter and a polarizing filter. I have looked on ebay at these and they seem really good value off there but i just wanted to know is it worth buying off ebay or paying the extra and buying from a reputable camera store. Any help would be greatly appreciated
A UV filter serves no other purpose than as a lens protector on digital cameras (film is quite sensitive to UV light, digital sensors aren't). Personally, I do use them because it's easier and much much cheaper to replace a filter than the front element of (or whole) lens.
One thing to remember though is that they can lead to flare and other problems. I've often seen people post images on this forum where they've shot lights at night and they've got a reflection of the lights between the front of the lens and the inside of the filter leading to a ghost image. If you know these things can occur then you can work around them.
Circular polarisers are pretty useful things and they're one of the few things that you really can't replicate in post processing later.
It is worth spending a little bit on these filters. Really really cheap ones are awful - I've bought a £3 polariser off eBay that does absolutely nothing apart from make the picture look yellow. Go for a reputable make and you'll be fine.