Just Chiming in regarding trades that do not involve a face-to-face
I'll preface this with indeed your mileage still may vary, and its up to you to seal the deal and go with your gut instinct.
I've done about 4 Trades over the post now of relatively high value RIF's
All of them have been trouble free
And following these steps has helped me, if these Criteria weren't met then it was Generally a Red Flag.
1. They have a decent seller history/activity on AFUK or another long standing Airsoft Selling Platform
2. They agree to send you a picture of their Photo ID (Drivers License) with a timestamp (a note with the date and name written on it next to the ID) allthewhile, reciprocating with yours, Bonus for UKARA.
(if you REALLY wanna be thorough, a picture of their ID, next to a Recent Utility Bill AND timestamp)
3. Then, checking postage address of the trade (including yours) directly matches the address of the ID
4.(Optional) one of the parties offers a collateral payment (the value of the traded item) which can be claimed if it goes pear shaped
You handle postage for your way.
additionally, as another user mentioned, proof of postage isn't really of any substance, but opening the parcel as soon as its been delivered on video will be. (also get the trader to send images of the item as they pack it as a reference image)
Yes it may come off as a bit thorough, but the trouble of losing an entire RIF with nothing to show for it far outweighs any effort in the preliminary.
And LASTLY Again, if the person you are trading with is apprehensive to send ANY of these details, then just dont trade.