Now that's a difficult offer to refuse! I, for one, can attest to Stephen's repair skills. freddyjollo's idea makes sense as well though, maybe you could could replace it with one not as nice (or as expensive) until is is fixed, then you'll have two working decks...
Your portable is newer than the deck you picked up. The deck only supports recording and playback of ATRAC SP. If you transferred files from a PC onto discs using the 520 it would have been in some MDLP (and NetMD) format. You could record something on the deck in SP and playback would work on the 520, but you will never be able to transfer anything via NetMD and get it to play on the older SP only deck.
I have six five packs of these unopened, 80 minute variety as well. I have often wondered their true value. It all depends on what the market will bear, I have not seen too many for sale in the last couple of years. There was someone selling 25 packs on eBay.com a couple of years ago for 180 USD, that seemed a little steep to me at the time.
They are definitely nice blanks, in my opinion the nicest Sony blanks I own.