Emile de Maat
1) In the search drop-down menu, there is a choice for "Accessories" and another for "Accessory". Is there an actual difference that warrants both options?
They are the same; there is no difference.
2) When I go to add an accessory to the database, there is no submission queue to check in case someone else already tried, like there is for boardgame/expansion submissions. When I do submit an accessory item, sometimes it shows up in the boardgame submission queue, and sometimes it doesn't. Any known reason for this?
Actually, the queue exists, but there is no link for it listed. You can find it here: https://www.boardgamegeek.com/items/boardgameaccessory/pendi...
Accessory submissions should always end up in the accessory queue - I have no idea why it would show in the boardgame queue.
3) Some accessories that are in the database and reflected in my collection don't show up when I try to list them on the Geek Market (for example, the Game of Thrones LCG first edition resin house cards), preventing the Geek Market from allowing me to list them at all. Other accessory items do show up and I am able to list them without issue. Is there a way around this?
The search function in the Geek Market doesn't support accessories. But if you first go to the accessory page, then try to list them from there, it works.