Sold boat, buyer claims problem, threatening legal action. Need advice

[rogue54]

What is the name of your state? sold in IL, on eBay, buyer from Missouri or Kansas.

I sold our boat on eBay Motors about 2 weeks ago.
I had the boat inspected at our local marina prior to sale, and had every recommended repair made. In addition to annual inspections and all repairs.
Boat in great working order.
Seller from another state purchases for $2200.
2 days after pickup, contacts me that the back end is rotted out, and it will cost $1200 to fix. Wants to know how willing I am to pay.
I had absolutely no idea anything was wrong besides what we had fixed. We had the boat inspected just before sale.
I told him I had represented the boat to the best of my knowledge and made every repair ever recommended, and had put $500 into it to have it ready for this sale.
Then he got very nasty and is threatening to sue me, to drag me down to his state and go after more than just the $1200. States that because I didn't state this problem in my eBay "ad", that I am liable (even if I wasn't aware?).
He scared me so badly with his threats, that I agreed to pay half, given that he sign a paper saying after this I'm no longer liable for anything that may be found down the road. I also asked for a copy of the repair estimate.
Then he tells me that the estimate was simply an inspector's opinion, and that he's not willing to sign a statement until he has the money. But he'll accept the $600 now as a settlement. Says he will take it to a marina for an "official" estimate.
Now I'm wondering if I'm liable at all?
I know that ignorance isn't an excuse when it comes to the law. So even that I didn't know anything was wrong, what's my liability?

[rogue54]

I finally spoke with our repair tech, who claims the transom was fine.
I'm also getting advice from people stating that since he picked up the boat at our repair shop, and tested the motor first, that he had a chance to look over the transom and it would have taken 2 seconds and since he's a dealer he would have recognized immediately if there was something wrong.
People are suggesting he's trying to extort me.
I'm looking for lawyers right now. Hard to find one that understands all this, since the winning bid came on eBay even though he paid in person up here. Hard to know where the jurisdiction is, if he really can drag me and my babies down to his state as he's threatening.