As im new to ebay..i sold an item and the buyer says his mate sold some stuff on his behalf on ebay and he still has the paypal money in his account..
is it ok if his mate pays and send the goods to him..

Sounds strange..

Any tips to avoid difficulties

In my opinion, don't involve anyone else, just deal with the Buyer.

Probably nothing untoward but you never know and it's better to keep things simple, especially as you've just started on eBay.