Those eBay asks are too high for used gear. The Polish ad is over new price, for a used unit. Forget the HiFi shark. Cary is selling new 300B SEIs direct for $3895.00, so estimate a fair used price accordingly. You didn’t say how old the units are or whether all of the original materials are still with the units, remotes, etc.
Any units that have been serviced, it is beneficial to have the service record, just as you would want with a car.
At this price, buyers will want to listen to them, or have a firm right to return for refund.
If you are offering the owner to buy them as-is, and you know they are working correctly, I might suggest 60-65% new MSRP assuming the cosmetics are excellent, the tubes are not too old and all the accessories are included, and less if there is less than that.