Where can i trade imei blocked iphones around the world?

Is there any site where I can trade an imei blocked iphone 4 16gb with an imei blocked phone in another country. I brought the iphone 4 16gb from gumtree and it turns out to be blocked.

To re-phrase your question:
Is there a site where you can trade Stolen goods?

and the answer (on YA) will be NO.

IMEI blocked = reported lost or stolen.
your only place to take it, is the network or manufacturer that sold it.
And they (and only they) may legally update or unblock the IMEI.

If you have bouight stolen goods,
or goods that the owner did not legally own,
then you should be reporting it to the police.

yes, on a friday, sat night many phones are left in taxi/cabs
ask any taxi company!
most owners don't know where they lost them and block them,
but that does not mean it is the finders to sell etc.