I would let your nose do the deciding.
Being fiberglass it should have been water proof and not absorbed the smells.
If it passes the sniff test bring it home and give it a good scrub down and you'll be good.
I hate smells! I passed up a great deal on a truck because it was owned by a smoker.
Definitely sound advice. I havn't been able to get in touch with the owner yet so it may be a moot point but I just wanted to know what others would have to say about the fiberglass and if it's absorbant.
I would hope the thing DOES smell... it would drive the price down a lot. Once home, I would clean and scrub it with Tri-sodium Phosphate and rinse thoroughly. After that, I might use some sort of dedicated pet odor cleaner, steam clean it, pressure wash it at the quarter car wash, etc.
If it STILL smelled, I would break out a small paint roller and coat the insides with either a fresh coat truck bedliner or even gelcoat. Either one should seal in the odor.
All that work just depends on how cheap you can get it.