all the details of what you should pay are available on the customs website, importing anything, new, secondhand, gift, posted or brought in through the airport in your bags is subject to import duty and VAT if its valued over the threshold. The duty is a pain to work out as you have to decide what description paintball stuff falls under. There are a few websites that will work out the duty for you out there (can't remember them off the top of my head).
What varies is the thresholds between gifts and purchases and posted or brought in in your bags.
However if you bring it in in your bag and its worth over the threshold value and walk through the "nothing to declare" channel and you get caught they could (probably won't but could) charge you with attempting to smuggle the item in to the country. There's notices all around baggage collection detailing your personal allowance so you can't claim you didn't know.
Also the customs people can pretty much decide what its worth, that isn't always what you paid, when my brother imported some bunkers he got hit for VAT and duty on the UK prices, not the ones he'd paid in the states (they were posted).
Yes there are ways round it, the warrenty claim, marking down the value of the parcel, producing a reciept for a lower than true value etc but these are all evading duty and technically criminal.