In the 10 years I've been using Apple products, I've never once had a battery fail me, and certainly not within the first year. If I could save $80 every time I bought a new iPad, but lost out on the warranty and had to pay to get my battery replaced if and when that happens, I'd save a load of money.
Agreed, I've never even seen an iPad fail within the first year, without any physical damage. If you go for this deal, just inspect everything careful, if it looks good, very small chance something will go wrong suddenly down the line.