Finally, something I know about!
From looking at the ad, a couple things pop out:
-That's definitely NOT a 403 Olds motor. This is a good thing, as the 403 SMALL bock (not a big block)has windowed mains and doesn't have the performance potential of the Pontiac motors. Which leads me to the next thing...
-That is a Pontiac V8 under there. Here's where it gets tricky: All Pontiac motors appear the same to the naked eye, from the lowly 301 all the way to the 455. You are going to need some casting numbers to figure out what that is unless you pull it and look at the stamping on the side of the block. Considering that there's an aluminum intake on there, it's likely either a 350 or 400.
-Unless someone swapped in a TH200-4R, the automatic that came in a 1979 T/A is a TH350. They never came behind a Pontiac V8 that year, either. It is a B-O-P bellhousing model, so you can just swap a Pontiac V8 in there with ease. That's what I did with mine. My 400 bolted right in with a few tweaks. The swap is done, so don't worry about that.
-The car is wearing 15x8 WS6 wheels. That is a good thing, but does not necessarily mean it has the WS6 package. Check for rear disc brakes if you look at the car in person. If the wheels appear to be original and it has drums out back, there's a chance that it has the WS7 package, which was a late production addition when they supposedly ran out of rear disc parts. You will have to measure sway bars, look for the little chassis braces on the front subframe that tie it to the body near the lower firewall, etc. Either package is a win.
-I have never seen a 1979 model with the gray velour interior. Ever. It may be a 1980 model. If so, no big deal. It's worn anyway, so whatever.
Now for the positives of buying one:
-These are hot right now. Lots of aftermarket companies are coming out with new restoration and modification parts seemingly every month. They make a ton more now for these than when I bought mine in 2002.
-The sky is the limit with these. They make stock resto parts as well as complete bolt-in subframes and modified suspension setups. And yes, there's plenty of junkyard upgrades too, but especially if it has the WS6 package, it has a lot of good stuff already.
-If you do buy one, be prepared for people to turn their head when you drive it and the inevitable Smokey and the Bandit comments.
-From my personal experience, I can tell you that there is no other car on the road or in history that has been clouded with hearsay and tall tales about what they really are than the late 1970's Trans Ams. Everyone turns into a GM historian when one pulls up.
-If you care, you can get a Pontiac Historic Services package that will tell you what your car originally came with, what options it had, where it was delivered and sold, and a copy of the original window sticker. Very cool.
Go buy it!!!