I'm very transparent about the way RGCD operates and the fact is that I make £5 'profit' per sale, around 10%. That basically pays for the time it takes for me to handle the payment and accounts, prepare and pack the game and contents, wrap and package the parcel, and take it down to the post office. Now, sure 70 x £5 sounds like a lot of money to make in one day... but in reality it has already taken all my free time (outside of my normal working hours) this week to pack and post 50 or so copies. I'm NOT complaining at all, but for me to work with a 3rd party would just mean more work for me with less reward - and it would ultimately end up with the games becoming MORE expensive for you guys, with the reseller adding a markup.
I have exactly the same issue with the C64 carts - I get approached regularly from people wanting 'trade prices', but the profit RGCD makes on each of those is even less (below £2 per sale).
Cartridges are not cheap, and hey, devs gotta eat...