I know it seems like a really great deal, but I think you may just want to refuse it.
$400 for a Lesser seems too low, well to me at least. The lowest I've seen a Lesser was at $800. But really, if someone wanted to sell a Lesser for that low of a price, there has to be something up with it. Sometimes people sell their animals for a low price because they think someone would buy it because it's low but the animal will most likely have problems.