With that car having sat so long, I'd suspect it's time to remove and clean/overhaul the carb. That Rochester has an Idle Air Control gadget that might be giving you problems. I don't know b/c I am not there to look. However, you say the car is flooding. What you say about the car flooding after running a while and over time not wanting to run at all is consistent with your float becoming more gas-logged over time. That is my guess as to what is likely to be wrong. It's only a guess and this advice is worth what you paid me for it ha ha.
MMmmmKay - consider getting yourself a rebuild kit or at least some gaskets, and a float. Let me explain. One thing that happens with the '70s and '80s carbs is the floats, if they are plastic, get gas-logged over time. That is, they are made of a plastic closed cell foam BUT they still slowly soak up gas. Some take (much) longer than others to get logged but they pretty much all do if you have them long enough. When that happens, the needle pintle rides out of its seat all the time and the fuel pump floods the carb with gas as a result. At the age of your car, if you've not replaced that float it is probably overdue; all you need
to do is replace them and adjust the carb, but what you should
do is overhaul the thing while you have it apart on your work table. There are a couple books on that, one is https://www.amazon.com/Haynes-Rochester-Carburetor-Techbook-10230/dp/1563920689/.
Bottom line for me is that what you say about the car flooding after running a while and, over time, not starting/running at all is consistent with your float becoming more gas-logged over time.
Right now the gasket kit will cost about $9.00 + shipping (rockauto), the book (used, Amazon) $7.00+, the float less than $30+ (rockauto).
On the carb that was on my dad's '74 Plymouth Valiant, it was a Carter or a Holley, don't recall which - the float could be replaced without taking the carb off the car - matter of fact, I did it (years ago) with hand tools in less than an hour. I admit that I put in a float from an older car, matched perfectly except it was brass. I think it came out of a '64 but I am not sure. I don't know about your Rochester, so I suggest you get that book. You may find your local/regional library has the book for you to borrow, actually.
As for selling your Riv, you can compare it to these https://classiccars.com/listings/find/1983/buick/riviera
- but what you can get for it is anyone's guess.