Paintball guns don't tend to blow up, like ever! at most its a loose fixing or an o-ring which needs replaces or extra lube for the bolt.
The only thing you can do when you're buying gear off of this forum is to check that persons feedback rating and other items they've sold recently.
With the Paintball community not being massive in the UK, if you do experiance a problem and the seller is unresponsive, people on this forum are usually nice enough to help out by getting in touch with someone whos knows the person who you bought the item off.
Ask the same kind of questions that you'd ask if you were buying a car;
How long have you had it for?
Who was the previous owner?
How much paint have you put through it?
Scratches or any other wear?
Leaks or problems in the past?
Last time it was services by the product manufacturer?
Spares kit?
You can always ask for a video to see it shooting before you buy it!
You're usually okay with buying in confidence of these forums!