As a career, I actually manage PPC (Adwords) and SEO for multi-million dollar clients. So, I have experience :)
Adwords is a WONDERFUL sales too. I myself have just started an Adwords account tonight with a similar voucher. For $75, you can't go wrong.
However, keywords in our niche tend to be expensive ($1.50- $2 per click), so $75 won't go very far. And, as a source you would pay for out of your own pocket eventually, I'm not sure it would give the return you'd need. But, that also does depend on the price of your items.
My advice is to give it a go. And be sure to set an end-date for your ads so that the credit doesn't run out and start charging your CC. If you're interested, I could do a blog post on initial set up "Do"'s and "Don't"'s for new users, if there's a request for it.
Good luck!
Andrea
http://StudioEleven11.Wordpress.com