I know that it is possible for Players to give karma to free spirits but is it also possible for PC's to give/sell Karma to other PC's

By canon? No.

-Siege

thanks

I think it is possible to split the karma cost of binding focui please correct me if i am wrong. along these lines would it be possible for another PC or NPC to pay some of the karma for a ally spirit?

Some players want there to be a way to buy or sell Karma. If a Gamemaster wants to provide an option for this, guidelines are described in the Shadowrun Companion starting on page 79 under "Karma for Cash" and "Cash for Karma".

We've never used such rules in our campaign.

We have, deckers cost a lot!

If you want to house rules such exchanges (and mages and others can give Karma to free spirits, so there is a precedent), then you could make it a metamagical technique. There could be specialists (like Blackjack's Karma Masters) who make a living doing such "exchanges", or a mage PC could possibly learn the technique (although it would probably cost a few favors).

IIRC Vodun shamans could steal karma in some way. I don't have my books with me, but it would be in MitS.

it's possible to split the bonding costs of a focus, but only between the crafter of the focus and its intended recipient. each karma point that the crafter pays into bonding it to the recipient costs an extra 5k nuyen. it's nice for adept and shamen, who have little else to spend their cash on.