I have 18 pages of cards, and I don't want to add each card individually.
There's no easy option for that right now (and am not sure if that's a scenario will be addressed in future updates).
If thinking about transferring your entire inventory to another member, I recommend that you do the following:
1) Create the trade and negotiate the terms.
2) Get the email address of the receiving trading partner.
3) From the Inventory page, export your inventory to a CSV file.
4) Email the file to your trading partner, and ship the cards as agreed.
5) You'll still need to remove the cards from your inventory, but you can do that a page at a time (use the check box to select all, and then click Card actions > Remove).