<--- fellow n00b here but I can field this one
The Auction House is kinda broken as you can't sell cards for less than the exorbitant minimum bids set by the game developers. To combat this, sellers will offer to buy back a common or uncommon (or whatever) card from you at above its value so you don't pay the full cost of the minimum bid.
The downside to this is that there is no avoiding the 30% tax on transactions, so the seller only gets 70% of the cut on both transactions.
e.g. Legendaries cannot be listed for less than 2000.
If I offer a legend at 2000 with a 1000 buyback, this means you bid at 2000, and then I buy it now your trash card at 1000.
I receive 1400 credits after the 30% tax for selling the legend, less the 1000 I spend on the buyback = net profit of 400 credits
You pay 2000 for the legend card but receive 700 credits back on the 1000 buyback after tax, meaning you just got a legend for 1300 instead of 2000 minimum.
As for training fodder, you need a lot of commons and uncommons to fully train a high level card. Often players at higher tiers are happy to sell cards they don't need for low level cards that are fully trained so that they can use them to level up high level cards.
e.g. I sell untrained legend for 2000 (receive 1400), I buy two fully trained ultra rares from you for say 1200 (in this case, above the minimum bid). I receive two URs that I will use to train a higher card and net profit of 200 credits for my legendary. You receive a legendary at a net cost of 1160 plus two ultra rares.