I agree with Vicki!
One thing I always try to remember, when standing at the gate looking at something I think is a lot poorer than what I was expecting, is that it nearly always costs more to feed up poor stock than it does to start with good stock. I say nearly because some places have tons of free forage and browse, and it is cheaper for them to feed "train wrecks." Also, if you get the skinny goats just dirt cheap or free, that is a factor. But if you are buying hay and feed, and paying the going rate, it's cheaper to buy well-fed stock.
That said, as Vicki and others say, be happy with your purchase, and it was a decent price. Also be happy with yourself for not being the rube he evidently thought you'd be, and for making him back down to a price in the realm of the real world for what you got. I think that shows well for you in the future.
There's a LOT of huckerism in goat land. A LOT. And ESPECIALLY ONLINE. So many folks are out to find that newbie to stick it to, and then run to the bank...and they KNOW the locals won't pay what they are asking...so online they go! Trust your own eyes.
I think you did fine, but for future reference it is a real good practice to never say "yes" if there is a "no" in you somewhere. That is how you develop a discerning eye and standards for what you will have on your own place -- and what you WON'T. If you do this, believe me, you will reach a place where as a buyer, no will come more easily from your lips than yes. Yes will only be said if it is exactly what you want or better, and the price you expect. When you get there, you may regret with hindsight that you didn't buy something, but you will never regret that you did.
I think she could probably raise and wean the kid with great supplementation and genuine goat minerals. It's good advice given here to early wean if you can. Or if you feel you must take the kid off right away, that won't hurt things and may help as long as you can handle bottle feeding, or sell it off. Follow the excellent advice here and it'll work out. Also, please be really attuned to her worm load and other potential problems like cocci that can strike hard in thin goats.
This is how we all have learned. Trust yourself, and know that the next time you peek over the fence at another goat, you will be the wiser for this one having come your way.