Can't buy something: You don't have some items required to finish the deal.

Migration in Progress: We are currently migrating the Hub to read-only mode as we transition to our new platform, The Forge. During this process, you may experience intermittent access or limited functionality on the Hub. Once complete, all new activity should take place on The Forge. We appreciate your patience while we complete this migration and look forward to seeing you at forge.sp-tarkov.com.

This is not an issue! Make sure you have checked the box at the bottom of the trade screen, or, make sure to click on "fill" button before willing to trade.
Also make sure that you have all the items in your stash, and not on your character stuff.

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