If functional, and if price is reasonable, go for it.
I have a Van Alstine preamp, I think the one you are looking at. Wonderful unit when functional, and as noted above, much better than normal.
If what you are looking at is like mine, the only concern is that the boards holding discrete components were buried in epoxy. If the unit has a problem, then fixing the problem becomes a nightmare. You can't test what is wrong until you dig. It can get really messy.
Check with Van Alstine to find out if these are the epoxied units. Check with the seller to see how the units are currently performing.