I'll admit I'm the one who's ended up with the Power Droid. I put a message of interest on the original post and things went from there. Whilst it's probably no comfort to anyone who was interested that MOC definitely won't be getting flipped once I receive it.
I'll also admit double-standards here - I've seen many items which have interested me on the Valuations page only for them never to appear on the Sales page, (usually due to a 'PM sent' message) which is why I put the 'would be interested...' message on there.
Whilst I agree that something needs to be done I don't think there are any easy or foolproof solutions. Whilst the regulars are likely to abide by any rules put in place it's still possible for a new poster to accept offers via PM or PM anyone who's expressed any kind of interest on the post. I don't think an auction is the solution, as these cause problems on some FB groups due to arguments over finish time, when they posted their bid, etc... Anonymous valuation posts is a possibility, but will need some administration. I think realistically all you can do is encourage people asking for valuations not to invite or accept PM offers and to have the courtesy to post their items on the sales page.