I have found this information on the official Playstation Forum:
"How to send a trade request?
When you exit black market to the game's title menu, the game will prompt you to post a trade request yourself. Tap yes, and you will be transferred to near. Tap the location update circle on the top right corner of your Vita screen. If you have already updated near less than an hour ago your trade request will be reserved for the next update.
How to find out that you have sent a trade request?
You can find out using near. You load up the app and navigate to the Discoveries section, and on the top left corner you see your PSN/SEN username and choice of avatar. If you tap your avatar a pop-up window will appear with a list of games. Uncharted Golden Abyss should appear on that list, with a gift box icon beside it indicating a trade request is in fact active.
Now pay close attention to the icon, if you see a green arrow overlaying the box, that indicates your trade request is pending and YET TO BE SENT. This pending request will be automatically processed the next time you update your location on near. If you tap on the box itself, you will be able to check the status of your trade requests.
How long do trade requests last before expiration?
Unlike some other game goods such as content for ModNation Road Trip that last two weeks, trade requests have a very short expiry of 3 hours. Be sure to claim your prize ASAP.
Do I have to wait at least one hour between updates?
In most scenarios, yes, unfortunately. However if you own the 3G/WiFi model and have an Internet connection either on your Vita or a mobile hotspot, such as a MiFi device or a smartphone, you could go out of your way and travel outside your local radius. Say near compares Vita users surrounding you in a 6-mile radius, you need to travel outside the 6-mile radius, and near allows you to update again within the same hour. I recommend doing this when you need to travel outside your radius for anything, including shopping, visiting a friend, or simply going to work. Do not use your Vita on the road when driving as it significantly increases chances of getting vehicle accidents. Use common sense.
If you're a passenger you have a greater advantage, you can update near every certain number of miles. The extent of your radius vary between areas. From my experience, your radius will be more extended in a large urban metropolis than in rural communities.
I have been troubleshooting this trade system for days. Me and a friend have been in a lengthy PM conversation over this whole trade system, having been perplexed, baffled and frustrated with this and ultimately we didn't know what to do. I was almost on the verge of contacting PlayStation customer support as most of the troubles come from the near app.
Then we found a theory that seem to have worked, but we need more information to fully confirm these as fact.
Only one person can download a trade request
This came up because many times I have uploaded trade requests the past few days, in which none of them appeared on my friend's near. Since I have multiple friends that are involved in this whole black market trade system one of them could have claimed my prize before any of my other friends could, thus could thwart any attempts to deliver the gift to a choice friend. Just today we recently scheduled a time (7:15 PM EST) to meet up on near, and apparently for once he was able to claim my trade request as he would be the first.
Yesterday I have sent out a total of four trade requests but he was not able to claim any, not even within their three-hour time window. Today was different, thus we believe this theory holds water. If the theory is true, you and a particular friend on your list have better chances pairing together at this against everyone else. Useful when wanting to deliver a good to a specific person."