Price is fair for a MINT example I'd say - stand & TV combined
Check the fret fabric is unblemished - no snagged fibres - as replacements are getting scarcer and prices rising.
MK4 will have WISA - check it works OK - if important to you
Check the TV interface - DTT, D-SAT or D-CAB - make sure it's suitable for your broadcast source
Check if it had the HDD option - and if so - check it can record OK
Manually tilt the stand to full left & right extents and listen for gear grinding
The back has a lot of clip-on / clip-off panel to routes cables - just like your 55 - the legs can snap off these if its had a clumsy owner. Not a problem unless the TV positioning afford you a rear view, in which case these panels can then look messy.
Only the ATV3 will fit inside - and even then it's a snug fit - the ATV4 and ATV4K are too big. If you have a lot fo source boxes, then the STB System 7 clamp stand works well.
Oh, and check what model of BEO4 remote you will get - there are various versions.
Listen for audio pops & crackles on startup - I have a 46 and a 55 - both from the secondary market (Thanks LEE!) and both have a mild audible click when the speakers get powered up during boot. For my MK3 46 this has definately 'arrived' since I bought it 3 years ago. The 55 - which is an MK1 - and hence older - already has it. Do you have this on your 55?
Generally, like you, I've found the BV11 platform absolutely rock-solid and an amazing audio master / video master. Picture is stunning, and when fed bluray, gives 4K a run for its money for enjoyable watching.
All things considered, prices are a steal for the 'experience' quality you get - esp. when you look at the price fo a new Eclipse or Harmony!
Hope it works out for you
T