I was wondering what this is exactly and how I can fix it. I'm looking at getting a used '99 Accord V6 and the CEL is on with this code. Thanks in advance for the help!

 

I wouldn't buy that car. 98-02 Accords with auto transmissions had major transmission problems. Honda replaced many, but a 99 model is now out of the 7 year free replacement window.
The car likely needs a new trans.

 

assuming this car gets a new tranny from Honda, would this be ok to buy?

 

The 6th gen trannies are hit-or-miss. I have one and I drive it pretty hard. I don't drive that many miles though. I've heard of the replacements not lasting as long as the originals and these trannies can have problems at any time. That said, the only symptom of tranny problems is that it hits R a little harder than I'd like when parking after some ripping on it.I'll be installing a tranny cooler soon to take care of that.

 

It won't be a free transmission from Honda.A 99 model is out of the "extended warranty" window.
A new trans will likely cost upwards of $3k.
If you put a new transmission in it, I would imagine it would be OK. But remember, Honda will NOT provide one for free on a 99 model.