Personally, I try to “grade” every coin I have in my swaplist, but I also notice that I am a very difficult person in terms of quality and an XF in NGC/PCGS would say be a VF in my own grading system.
I like the idea of having some “value” shown in the swap list that's outside the comments section, I use the public comment section for variants or if some coin is reserved. I already price my personal collection (singles) for what I WOULD SELL THEM FOR, not WHAT I BOUGHT THEM FOR. Because I buy world coins per kilogram mostly and cross reference what I would sell my entire collection for, if that time comes.
Anyway, if you can give potential swappers a quicker, easier overview what you would ask for every coin? That would be a great idea. If you can add a personal value to swappable coins and it counts that, just like the amount of coins added to said swap? That would be an amazing feature! And mandating a quality would be great too, even if some people don't “grade” their coins.
But I usually do a swap that consists of “x coins for x coins” or if I truly think a swap is “out of line”, do I check the value on Numista; even if the prices aren't completely fair, it gives an indication and that's all that they should be in my opinion.
Honestly? There's no conclusive website that gives a good value for coins; I know some coins are more valuable in other parts of the world/more difficult to find, so I guess that's the great thing about swapping here or on any other site; that's for the parties to decide. Hell, I keep getting people who want a lot of Netherlands Antilles from me since those are difficult to find for them. I get them very easily in world coins I buy on Marketplace and the like.
Some people think all of their coins are made of gold, others are very easy because they only swap duplicates; both is fine. Some swaps won't work out, and that's fine too, not everyone has to go through the same door on friendly terms.