There is no direct way to convert the karma into coins as all the items you trade with karma are immediately soulbound on acquire. However there are some important uses for karma.
If you ever want to level your cooking you will need to save some to buy recipe items. Although the cooking items aren't worth much on the trading post, the magic find bonus will give you better drops and the stat bonuses could effectively keep you alive when otherwise you wouldn't have. In this way you indirectly make money by not having to fix your armor. A copper saved is a copper earned.
Another important use for karma is to obtain recipes for crafting. With the exception of cooking, you need to buy recipes from all the master crafters in order to reach level 400 crafting. The crafting only reveal insignia/inscriptions up to level 350. Once you reach about level 380, all the recipes are grayed out and you can't earn any experience unless you use the recipes from the master crafter that you need karma to buy.
You can use it to buy armor and weapons and accessories from karma traders around the world to supplement your gear if any of them fall behind.
Please note that you should not use your karma to buy cultural weapons. The cultural weapons do not go towards any title and is only rare not exotic so it is not max stat. Since they cost 63k each, it is not an effective way to spend karma.
However outside the Gate of Arah, you are able to buy maxed Exotic Armor pieces for 42k karma a piece. I find this the most effective way to spend your karma. As of now, a piece of Exotic armor is equivalent to about 2.5g gold coins. If you save up the karma you can buy a complete exotic armor set and save about 15g you would otherwise have to directly spend.
Best Answer
There's not a huge amount you can do with these after you "outgrow" them, apart from:
Otherwise, there's no point keeping them and you can't sell them to vendors (because you can't get karma from a merchant, and if they gave you coins for it there'd be a way to directly convert karma to gold, which ANet wouldn't want) or salvage them (because you could then sell those proceeds, which ANet wouldn't want). With the recent update, you don't need to keep them for transmuting their skin either, because the skin has been automatically unlocked on your account when you logged that character in after the update.