I have my eye on a set of Forester XT struts, 04. One of the top hats is cut up as he had difficulty with a stud. My current top hats are all in good shape. Would I be able to re-use one of mine, or would I need to buy a new one?

 

My car's a 99 Legacy Outback. 

Yes, the should swap over.  Sometimes you will run into spring seat issues, but you will not know for sure until you have the struts out.

 

Sometimes I have to put a washer under the nut to make up for the longer stud on the strut.

Thanks. 

 

What's a reasonable price to pay for them? He says they're in good shape, no leaks. I'd be picking them up so no shipping. 

If it's a front top hat then yes, if it's a rear then no. Luckily all 04-07 impreza and forester top hats are interchangeable and fairly cheap to buy new.