In this area you can get nicer rigs for less money, and most of them are used seldomly. Look at the texarkana and little rock craigslist. Maybe even fort smith. I've bought several from fort smith craigslist and have always done well with them, most recently a 1542 Grumman with a 25hp 4 stroke yamaha for $1200. Picked it up, got home about midnight, went fishing at 0600 next morning with nary a problem other than I didn't really care for how it was laid out.
If he's only used it twice, he'll probably have to address old gas. Depending on how badly the old gas has penetrated and what motor it is, may just need a carb kit/cleaning or it may need a VST pump & injectors (carb vs efi). If EFI, there's a possibility that it may get real expensive if a dealer/tech had to be paid to do the work (parts + labor).
To me that's a lot of money for a used rig, I don't care if he's used it twice or two hundred times, it's still used. If no hourmeter, there's no way to know how much it's actually been used. Lot of folks have complimented me on my "new" boat. Mine's a 2012 and has been used a LOT, I just take care of it. I see/hear this all the time and the most recent one is a barge I'm working with that has a F115 on it. Guy said he bought it and it dont' have 40 hours on it. I hooked it up to find that it's got over 1400 hours, and it shows how many hours it was used from 0-1000 rpm, 1000-2000, 2000-3000, etc. About half under 1000, the other half above 5000. Thats the nice thing about yamaha. I don't know of tohatsu/mercury does that or not. Had a guy bring a bass boat by a couple weeks ago wanted to get the hours off of it. Seller said it had a few hundred at the most. Turns out it had 27 and 24 of it below 1000 RPM (F250). Buyer paid the seller on the spot, $18,000 in cash.
Thirdly and finally, a 1648 seark is a heavy well-buit hull. A 20hp on that hull is a pig. I'd be surprised if it'd see 20 mph max with that tohatsu/mercury assuming it will even get on plane. In this area, it'd be lucky to fetch $3500.