Hello
Have a look at the second hand thread and dont buy the 1st thing you see. Have a look and read peoples descriptions and get an idea of what is out there and the price compared to the condition. There are loads of items that come up on the for sales threads like markers, goggles, hoppers, air tanks if you miss one another will come along and if it does not p8ntballer has a wanted thread section.
If the person is not local to you and they have to send it in the post and you have to pay via paypal, it is not worth taking the risk and sending money owed I would pay the fee's so it will give you some added protection. If you have any friends on P8ntballer and you see a gun up for sale ask them what they think about it. Its worth doing your research.
If you want to buy new and depending on where you are in the UK there may be a paintball shop near by or a drive, you could go and have a look and buy one of their starter packs.
Some of the starter packs may come with CO2 cylinders and others with compressed air, just check with your local site what they use or otherwise there may be more expense in getting another tank for you gun.
thanks