In the Peter F Hamilton book, Reality Dysfunction, there is a very good example of rare goods trading. Peter is English, aged 57, so Elite will have been an influence and inspiration to his sci-fi work.
Omitting one or two details from the book, here is a rough summary of the parts relevant to rare goods:
The planet Norfolk is a low tech planet that produces a valuable rare good, the liquor Norfolk Tears. The Tears are only sold during the annual harvest and are mostly bought up by large syndicates. A percentage of the harvest is reserved for casual traders, which keeps the price high.
Two of the principle characters, Joshua Calvert and Syrinx are starship captains who arrive at the harvest in the hope to get some Tears. They have each saved plenty of money, Syrinx through military service, Joshua through smuggling. They have both also brought rare goods with them to help secure a load of Tears.
Syrinx, along with many other hopeful Tears traders, went to a waterworld and bought valuable seafood. Syrinx did fairly well for a new Tears trader, she secured 200 tons of Tears, which was ½ what her starship could hold. She had committed to sell 10% of her Tears cargo to her seafood supplier, so had to return to the waterworld.
Joshua Calvert took a risk with the rare good he shipped to Norfolk. He guessed that an ultra-hard wood might be valuable on a low-tech planet like Norfolk. So he found a colony world that had the hardest known wood in the galaxy and filled his 1000 ton cargo hold with it to take to Norfolk. To get the wood, he had agreed to take another character as a passenger (though he ended up taking the book’s principle villain). The wood was indeed wanted on Norfolk. Joshua also agreed to take an injured relative of his Tears supplier to a high tech space station for a transplant he couldn’t get on Norfolk. Joshua managed to obtain 1000 tons of Tears as cargo.
In game terms:
Syrinx bought a 400 tons of rare seafood by taking a mission to return with 10% of her cargo at cost. She secured 200 tons of the rare good Norfolk Tears.
Joshua bought 1000 tons of a little known, but not particularly valuable, rare wood by taking a passenger mission (later updated). He took another passenger mission from Norfolk. He secured 1000 tons of the rare good Norfolk Tears.
This could all be fleshed out in the game using existing mechanics. Certainly beats just waiting 10 minutes for maybe getting another piddling allocation.