In our country, we have a forum called AquaplantExchange it mainly is aimed towards swapping and or selling bits of plants the members grow... It often is a demand thing, loads is offered but never sold and finally ends up in the bin since the majority of the hobbyist do not have the means and material to keep it indefinitely... A simple construct is, offer it cheap to make it sell quick to somebody curious about it. Just not curious enough to pay the top price.
Do you want a top price you have to wait for that one person feeling in need to want it this badly. That's back to square one, how much and how long are you willing or able to keep it fresh and alive for that? Investing in this would make you want even a higher (reasonable) price for this.
Reasonable is a very personal thing... Reasonable for you could be far from my budget etc. And it can be hard to compete with other sellers having a different and cheaper idea of being reasonable.
Next thing, P&P is often a bummer... I don't know about UK prices, but for me, an envelope with a number 1 stamp is about €1,- and with this, I can send 60gr in a letter. 60gr moist plants securely packed ain't that much. 60gr to 90gr will be 2 x number 1 stamps. The moment the size exceeds letter format P&P becomes €7.95 and this is no longer lucrative for a single cup of Fissidens moss. Then a trip to the LFS is cheaper.
Since it is perishable life goods selling small bits cheap and quick is the next best option. Then you at least have a little in return or else it ends up in the bin anyway.