Before the Conclave Arcana launch I bought about 80 alchemy packs in the presale. I got lucky and pulled a bunch of black foils including a legendary (Palifala Weslinath) and an archon (Razi Clanktrap). Since then I have opened more packs and pulled a few more black foils.
The market currently values the legendary at around $1500 and the archon around $990. I believe this may be under what they are actually worth due to the low mint numbers of both cards. Only 7 of the legendary have been pulled so far and 31 of the archon.
It's hard to put a true value on the cards as few have been sold on the 2nd card market. Peakmonsters for example has no data available for most previous sales. One option could be the new auction site Ahonex.com which seems to be operating smoothly. I have previously bought a few cards via this auction method. I can set a reserve price on this site too so they won't sell for peanuts.
I currently have all my black foils on wagons ready for the upcoming conflicts. This is another unknown as I don't yet know how many of the legendary cards my wagons will earn me. I think I have above average wagons but could still get blown out the water by a bunch of whales. Another option is of course to put them on land as they have such high production points. I am starting to increase my land production again as I build up my staked DEC. I used a lot of this DEC to buy the conclave arcana packs
In conclusion, I think my best bet is to wait until at least one month of the conflicts is over and see how many cards I earn. Then I will reevaluate my options.
Vonak!