Radford STA25 Mk 3 modifiactions
We have apples: "Good working order".
And we have Oranges: "Of course it's a very old amp...".
That it was only a capacitor is remarkable.
That the amp was modified is, likely, a contributing factor.
That it is not the seller's fault is also very most likely the case.
I will lay dollars to Krispy Kremes that the seller ran the amp for 20 minutes or so, and had no issues. That, in eBay speak is "good working order".
And, for the record, had that been my amp, it would have spent its first 8 hours or so on my metered variac, being watched very closely. Likely, I would have seen the rise in current prior to the cap going *POW*.
This is why, when introducing a new component into the herd, a certain amount of precaution must be taken.
If you have no skills personally, this will be costly, because the 'proper' fix is to restore it to its original condition. If you have such skills, less than US$100 should more than cover it in parts.
Best of luck!
Melrose Park, PA