The basic principle is buyer beware but as there is a legal requirement to maintain the rifle within the legal limit, then it seems unreasonable that the original vendor did not A, check the rifles energy level to ensure it was legal during the period of his/her ownership and B. did not take this action prior to selling it.
Purely by chance, I tested my S200 last night over a 55 shot string. When I last set her up, it was peaking at approx. 11,5 lbs energy. When I checked it last night it had dropped 1 ft pound !!
As a result, it has been tweaked and is back to 11,5. IK will testing it the next time I shoot as they have a habit of settling down over a few days.
Access to a chrono is not that difficult even if it means taking it to a shop or joining a club. Combro produce small portable and very cheap and accurate chrono's though I hear they are rarer than rocking horse droppings at the moment ?
You say that it is exceeding the legal limit every other shot but you dont say what the average is. If it is running at 11.99 ftlbs then it is clearly too close for any reasonable allowance due to pellet weight fit temperature etc...
However, you mention that the reg has to be replaced. Difficult one to answer as the previous owner didnt seem to be too aware as to what his equipment was doing.
Morally, I would offer to meet you half way but then again I wouldnt have been ignorant of my rifles performance in the first place.
You may have some leverage as he sold you a rifle that was exceeding the legal limit and therefore may have breached the law IRO firearms legislation and possibly IRO consumer legislation.
Suggest you contact your insurer as they often have a legal advice line as part of their standard policy cover, and most brokers offer legal protection so you will get advice and they will issue proceedings in your name should the case warrant it.
You could also make the sale conditional on the rifle being legal and in the condition it was described. Ask if it has been serviced - this may give you an insight as to the attitude of the owner etc...
Steyr
In a battle of wits I refuse to engage with an unarmed person.
To one shot one kill, you need to seek the S. Kill only comes from Skill