you are not alone.  I started a thread about this very same sell2buy promotion some weeks back. 

http://community.ebay.com.au/t5/Selling/anyone-else-get-this-eBay-selling-voucher-email-25-for-every...

I was told in an eBay message that I should go ahead and buy $75+ worth of items and then let them know the item numbers to receive a $75 PayPal credit.  I did buy and pay for some items, let eBay know the item numbers, and they turn around and tell me that in 15-21 days I will receive my $75 PayPal credit. That was on the 27th July.

It has now been 16 days and I will be contacting eBay in 5 days time if the credit has not arrived.

So much for the voucher arriving on the 11th July!!!!!!