Is there a way to view previous purchase ad/description?

To the best of my knowledge there isn't.

I know it doesn't help you now, but I always save the webpage for an item I'm about to buy

If you go into your personal settings and look up purchases/sales, it should bring up what you bought and bring up the original ad.

Yes -- if it's your purchase it's under your "Bought" list, look at the "original listing" tab

Also if it's someone else or something you remember seeing search via HiFi Shark and look at sold listings, you can pull up descriptions for expired AudioGon listings, the pictures might not be there but the description is retained