You could always try and see if they take or not. Some folks do it with tomatoes and squash, so why not try? Leaving them to further ripen on the counter won't really do anything for seed development if they aren't already mature.

There are also seed companies that sell a goodly handful of Asian produce like eggplant and bitter melon. Bet someone has wrinkled cucumber too.

Thank you, that is what I was wondering. I will see what seeds I can find out there. Once I get a crop, i'll leave one to go over-ripe on the vine :)

For the eggplant, it is unlikely that you could ripen store-bought fruits to the point where the seed would be viable. At harvest stage, the fruits are too immature. To get seed, eggplants need to ripen on the plant far beyond the point of edibility.

For bitter melon, it might be possible. I have seen store-bought bitter melon that were very near to maturity. You could allow them to ripen further, but they will spoil quickly. If the seeds are surrounded by deep red gel, and the seeds within have begun to turn brown, they might be viable. If so, those seeds would probably only be good for the following season.

Don't know your location, but seeds for both are widely available in the U.S. The long, light purple eggplant most often sold in Oriental stores is Pingtung Long. One source is here: Pingtung Long

The same source also carries several bitter melon varieties. More bitter melon seed can be found here: Kitazawa seed

If you are in the U.S., I could send you bitter melon seed when the weather warms, I grow & collect seed for several varieties.