[Monica]

Is it a good idea to introduce another bird into the house at this point?
Macaws *can* be sensitive to the dust of african greys, cockatoos, and cockatiels. Is it a good idea? Are you prepared for another bird? Do you have the room? The space? The money? This may be something only you can answer.

What are the chances of Zuri and this African Grey getting along?
There's no telling. They could like eachother, they could hate eachother, they could ignore eachother, or perhaps one will like the other but the feelings are not mutual. You are asking for personal information on animals that even if we knew them we couldn't tell you what the results would be.

What are the chances of them fighting?
See above.

Any tips on introducing a new bird into the mix?
Your best bet is to first quarantine the grey, then once out of quarantine, allow the two species to hang out within neutral territory, away from the cages. Put the cages in the same room but not right next to each other so that the birds can become accustomed to each other.

Also, I know that many of you have taken in birds before. In a case like this, would you offer full asking price? Try to talk him down? Offer to take the bird, but not pay him?
That is all up to you. Also being able to see the condition of the bird would help. Did he buy the african grey second hand, or from a pet-store with all new supplies?