site night vouchers on eBay - terms &

Hi

I have been looking at site night vouchers for sale on eBay. Can anyone tell me what are the 'terms & conditions' that are on tha back? Are there general restrictions? And if I stay on a site with different price bands for different nights due to the time of year, from which days will my vouchers be redeemed against by the wardens - the cheapest night, or whichever nights I nominate (the expensive nights!). Just trying to work out if they are worth buying? thanks