The first thing you need to know is these cars are not all that rare and not all that collectible. Cars made after 1972 aren't in the same muscle car category as those made before and shouldn't be priced the same. One year makes a difference, and just because a 72 Road Runner can sell for $25,000+ doesn't mean a 73 does. But, a lot of sellers out there believe Old Mopar = Big $$$$ so beware.
Don't let them run the price up on you by saying it's a numbers matching 318 car. There's no real value added when you're talking about a 318 car.
Be sure to check the lower portions of the quarter panels thoroughly! There's no such things as "a little rust" in these areas. Poor weathersealing and drainage in these cars caused a lot of rust to form from the inside out in these areas, and if you can see a little on the outside, you've likely got a lot behind it. Also, only the areas in front and behind the wheel well are reproduced. If the rust is all along the quarter panel, you're going to be looking at replacing the whole panel and that gets real expensive but quick! Rust on the doors or fenders isn't such a big deal, but I pass on cars that have a lot of rust in the quarters.
Check around the rear window trim for rust as this is another area that tends to rust out. Trunk floors, seat pans, and the space between the windshield wipers are also prone to damage as well.
Check the first two characters of the VIN. They should be RM. If they are RL, RH, or RP, the car is a Satellite and not a Road Runner. A lot can happen to a car over 40 years, and many Satellites have had Road Runner parts and markings put on them by various owners, and over time have become known as Road Runners, but if the price is based on the car being a Road Runner, make sure the VIN is RM.
You didn't mention a price, but here's what I go by for a 318 car: Mint/restored: $12,000-$17,000, very good condition: $8,000-$12,000, daily driver (minor rust, decent paint, runs good): $2,500-$8,000, project car (rust, needs paint/bodywork, mechanical issues, major interior issues, etc.): $1,000 to $2,500, and parts car (severe rust, major problems, no drive train, etc.) $300-$1,000.
Lastly, there's no such thing as "there are a few minor parts missing". As I mentioned before, 1972 and earlier cars are a lot more collectable than 1973 and later ones, but many of the parts on a 1973 also go on 1971 and 1972 cars, which means the prices for these parts are based on demand for 1971/1972 parts. Replacing something simple like interior panels, seat belts, even ash trays can run up your costs. Find out what's missing, and adjust the price for it.
