If I bought a Samsung galaxy S3 from gumtree, and later on the person i bought it from decided to not pay his bill, and phone gets blocked, do i have any legal way of getting phone unblocked, (i.e, showing receipt to network etc etc)

I do not want to pay £300 + for a phone and find out owner reports it stolen or just does not pay there bill resulting in it getting blocked.

Buy one from a reputable supplier then. Do you really think a recent model phone offered on Gumtree for ca$h is going to be legit?

Is there really any need to call me an idiot.

My post asked for any legal advice, not asking for some one to try to belittle me. I was hoping for someone to post with a sensable reply.

Where's the word idiot ?

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I am certain that you won't be able to use it even with receipt cause I'm sure once zapped they can only be reset by a change in imei number
Maybe the dr can enlighten us all

Can you not see the smiley

If you buy a phone in a one to one purchase (Gumtree, going to house etc) and they don't pay their bill, or report it lost/stolen to the network then it's going to get blocked.
If they have reported it missing and you contact the network and can show that you bought it from the person who is claiming to have lost it, then they'll look at it a bit more closely, but if they haven't paid the bill then you're stuffed.
Changing IMEI numbers is illegal, has been for a few years now, and doing it, even if it could be done, a lot of phones can't be, will find you with room and board for up to five years.
eBay has one redeeming feature in that if you pay with PayPal and it gets blocked you can file a claim within 90days and get your money back.
The phone itself is YOURS, you can do with it as you wish (apart from change numbers and working on the basis it's not stolen), but the network owns the IMEI number (a bit like DVLA, you can buy a personal Reg number for thousands, but if you change it or mess around with it DVLA can take it off you with no recompense).
If it does get blocked, at this time, it is only blocked in the UK, so selling it elsewhere in the world (which has its own risks) or breaking it down for parts (screens can be VERY expensive) are pretty much your only options, unless you just want to use a phones other features and not make calls.

Hope this helps you.

if you buy a mobile sometimes people have upgraded and sold old one or preferred old one and sold new one, i don't like my smartphone so may sell, i guess its best to get proof and a receipt and make a judgement on gut feeling also.