buying out is an option usually opted for when one or both players are:
Not playing well, and one is ahead by say 4-0 or 4-1 going to 7,
emergency comes up, roads gets bad, time to go home ect......
I got edified on buying out when I was playing a guy for $100 race to 7, he broke in the 9 twice and slopped in the 9 before I got up to the table, finaly he missed score 3-0, laughing he asked me if I wanted to "buy out" for $50, which I didn't know what it meant, he explained it to me, and before he was done, I was laughing at him, and asked him if he wanted to settle with me for $50, not that I needed to shake him up to beat him but he could barely get the energy to kick at balls for the rest of the set, I won 7-4, then I asked if he wanted to double the bet and I gave him 2 games, 7-3 with the 2 game spot. Not important, but a cute story non the less.
don't settle if your at the end of a set(unless you feel shacky, and unsure of your self) if the score is 5-0, 5-1, or even 5-2 going to 7 I wouldn't, expecialy if you and your opponent are equal, or even if he's maybe a ball better, winning breeds confidence. 5-3 depends on opponent, and who's breaking.
If it's with a friend, well all advice is out the window, good luck with that one.