usually when you start asking yourself if you should sell your car, the answer is yes.
i had an 83 240 stick which is the same as yours (B23) that i disliked very much, but it was a poorly tended piece of junk. the 240 is a great car, if you can find one without rust, and starting in 1985 uses the same motor as the 740. many would consider these 240s "classics" as i see them listed for a couple grand around the internet, and they are volvo's iconic model. the 740 is nicer to drive and a little more refined but was not as popular, so i don't necessarily know they have as much value, especially in the cosmetic condition you describe.

basically, i don't see it gaining much value without you dumping a bunch of money into it.
i just bought my first non-volvo (2002 VW jetta TDI that i'm going to be running on vegetable oil). it was a little tough, being that i've owned 6 volvos, but sometimes you gotta make the switch!