It will be 40 $ and this is only to avoid scammers from our forum , here in this forum marketplace is free and there are full of scammers !
We will try to make the best thing to avoid scammers in our forum
So, why would anyone pay $40 to use the marketplace of a random forum? How does giving it for free to 30 random users make it any safer?
And why do you think a $40 barrier is a good indicator that there will be no scammers? Everyone should use an escrow for their trades, and it shouldn't be different for your forum (that has an disadvantage of costing an extra $40 for no extra protection at all).