Gotta share my toughts on this, because I've been thinking a lot about it before.
I'm seeing a couple people saying that implementing BP trading would make people contributing less for GBs, since they wouldn't need to do it for the BPs anymore. For one thing, as I see it, more people contribute for the medal/FP gain or guild interest, then for the BPs, especially those who already have an 80Arc. Beside that, a lot of people contribute as an exchange of daily FP harvest. I believe BP trading would hit non of these negatively. What's more, I believe that maybe even more people would contribute, since they would be having a GB ready to level up, without the need of hunting for BPs and spending FPs on them. More people could just exchange their daily harvest and make their GB grow by it. One could skip spending FP for BPs and just spend them on the leveling. All in all, the whole process would be cut a little shorter, which I believe is couraging to building rather than discouraging. Of course, it's only how I see the game and the GB stuff, it can be different for someone who plays a different way than me.
Now for the trading part itself. First of all, trading as a whole should be fixed once and for all. There's not a possibility to make trades for non-guild members only. Ever since this game is running there was need for it, and we never got it. And ever since this game is running there were conflicts that could have been prevented by using it. (So you could happily trade 'non-fair' - and it's quoted because fair is relative - with your neighbours, without fearing that your guildmates will accindetally accept them. Maybe so that guildmates can see them, but can't accept them, sort of a safety thing.)
Once this is implemented to the goods trading, the BP trading should be the same. But instead of the little picture of the good, there would be 9 dots or small squares aligned in 3x3, 8 of them are dark brown, 1 is different color (something that fits into the scheme of the game, light yellowish brown, I guess) to indicate the place of the BP. And instead of the name of the good, there would be the name of the GB.
And if you still believe this would be bad for the GB building economy, add the possibility to trade BP for FP, with a limit of let's say 1BP:4FP (based on that you can buy 1BP for 200 diamonds and 1FP for 50 diamonds ), and I guarantee that a lot of 80Arc people will contribute to as many GBs as they can.
On a more realistic side, this is probably not gonna happen, simply because you can buy the exact BPs you want for 200 diamonds each and direct trading would cut this income for Inno. But an exchange to a random reward, like the Wishing Well's, would be nice too.