jdwaddle; Mr. Waddle lets look at this objectively
Since you seem very aware of the prior ownership of the gun, you have an advantage as a buyer. A round count of 5000 is very low for a MX8 and that's a good thing.
Lets look at the market value of the gun if it were broken down into parts.
#1 A Mirage receiver and forearm iron will bring $1500.00 EVEN IF the numbers do not match
#2 The MX8 trigger group - if its a selectable group, it will bring about $1200.00 non selectable $800 or so.
#3 The 27.5" barrels are not very desirable in the open market, but would have a value of at least $500
#4 The Briley Ultralites - while short in length DO have value as does the Americase container - I would rate the combined value at $750.00
#5 The stock - if its a Perazzi stock - standard grade, one can usually expect $400. 00 or so.
I would place no worry to the comment concerning the inertia triggers
It seems the current owner has been shooting tubes without any problems. Besides removing the inertia block is a minor fix to begin with. So this puts you at a max value of $3950 minimum value of $3550. Good used 32" Sporting barrels are easily found in the $2500.00 range and as far as barrel fitting goes, contrary to some opinions, a very high % of P barrels interchange without any additional fitting at all.
It appears to me that the seller has the gun priced correctly as a whole unit, you will need to decide if its the correct gun for you.
As a side note, one can usually find a clean MX8 with 31 1/2" barrels in the $5500.00 range and even less if patient!