I've listed a couple of fully working Plus-D interfaces on ebay

Some ZX 81 games

Includes the following titles as published by Sinclair for the ZX81.
Tapes are used, and sold untested.

1. Chess (Psion)
2. Blackgammon (Psion)
3. Flight Simulation (Psion)
4. Sabotage (Macronics)
5. Inventions 1 (ICL)
6. Club record controller (ICL)
7. English literature (ICL)
8. Spelling 1 (ICL)
9. Music 1(ICL)
10. Geography 1 (ICL)
11. Mathematics 1 (ICL)
12. Biorythms (ICL)

Plus (Free) English Literature 2 (ICL) published by WHS.
All in good condition, but sold as untested as I have no ZX81 to test

And some spectrum games at eBid:

Galletron game
Machine code for beginners
Mugsy game
Disco Dan game Spectres game
Quattro 4 - Power game (Codemasters)
Paris to Dakar game (sinclair)
Paris to Dakar game (Codemasters)
Super Seymour game (Codemasters)
Super Seymour game (Codemasters)

Lots start at 10p, postage at 1 with discounts if you bid on more
than one tape. I'll be posting some more games soon.

I will now re-lurk