Back in the days, there were secure trade option to make a safe transaction between the players.
However, this function was broken and unfortunately never fixed
The only way to 'trade' nowadays is indeed selling card through private sales.
I believe secure Trade Option was closed because according to Chaos in a post explaining about Secure Trades, there was an exploitable bug that allowed duplication of a card you owned as a result of using this.
Personally when it comes to Private Sales, my general rule is that I look at the player`s profile page for general information on them regarding the trustworthiness of the player and some of their public forum posts. I do take the time to check their collection ranking and cards they have put up in sales and make a quick decision to determine if they really have the cards.
If a player is a friend who has done a past successful trade with me, I go straight into it. I always double check and send PMs to verify details of what is to be exchanged to ensure there is a mutual understanding of what we is being traded.
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If you are trading cards and a player buys a card in your PS and does not do the same by letting you buy a card in your PS, you can remind them via PM to uphold the deal.
If it takes more than 24 hours, I can suggest contacting Customer Service and show them info on your Sales Thread, Purchases and Sales History as an indicator.
Rather than fixing that feature, it was discarded entirely. I'm no coder but it doesn't seem that complex to fix something like that in order to create a safer trading environment for the players...
Thank you for all the responses!