First off, you need to abandoned the idea of using online book value tools like Edmunds for older car values. Anything over 10 years and/or specialty vehicle will not be correct, and err on the side of very low. Remember, they just average sale prices, including all of the beat to shit/rusted out/auction nightmare ones to come up with their values (or some fancy depreciation algorithm that is generic.)
Second, Ebay and cars is an ok place to get a rough idea about sales, but remember you need to reach the buyers who are interested in it within 7-10 days, or you don't get the bids.
Third, You paid $7000 in 2007 for a 12 year old convertible, which was a good price. It is now twice that age at 24 years old, so I would not expect it to be $7000 still. It is still in the depreciation stage, and has not started to gain interest and rebound. I would not however believe Edminds again that your car is $2615 dealer retain If your car in excellent shape was on a dealer lot, it would be priced $4500-$7000. (I know nothing about your car, so I am assuming a excellent shape, 1995 vert, no mechanical issues, under 100k). Especially now in the spring.
These cars have started to move into the niche market area, meaning that to the general public, they could be not worth much, but you are starting to see people who want them will pay for them if they want it. They have not bottomed out and started becoming collectible items for the masses yet though, so prices will still vary.
Now, back to the car your brother was looking at. It looks like they fixed the electrical issue, or decided to remove it from the ad: https://cleveland.craigslist.org/bfs/d/independence-1994-olds-cutlass-supreme/6846110623.html
Given the price is now $2500, there will be little room to make a big profit on it with sales tax and DMV fees added in. At $1500-$1900, I could see a $1000+ profit coming fairly easily, but not quickly. First off, better pictures would help sell the car, along with cleaning it up and removing the seat covers, which they state the seats are good underneath. An oil change would help. Most of the suggestions here seem to be items that you would want to do if you were keeping it, and are good things to note and to look at when buying, however I am sure those can be left to the new owner to manage. When flipping, you want to take care of the current issues, not necessarily the potential issues. Aside from potential unforeseen mechanical issues, he would have to be willing to hold it for a while if he wanted to maximize profit. Pricing it at $4250 to start and taking it to cruise nights with the top down and getting it detailed would help put it in front of people. I would guess it would sell at $3000 to the right person.
My last piece of advice though is, if they are unfamiliar with the car, and the margin is low in this case, it is probably not worth it. The best flips are when you have a buyer lined up, you can price it low enough to turn it in less than a week, or you are OK with keeping the car if it doesn't work out.