What bothers me is the spread of prices for a LFH gearset. One has to wonder if , as you note, there are seconds on offer.
Probably the best bet is to go with a reputable dealer who will allow you to examine before you purchase and accept returns
after you buy.
Old boxes in good shape are not easy to find. If you look on ebay you will see most have pitted teeth or corrosion due to sitting
and collecting water over time. I've had several Triumph 5 speeds and only once, back in the early 70s, did I have a gearset
show a problem. That was third gear faces showing hairline cracks in only about 10k miles. Replaced with a used pair and
never had a problem again.
Breaking a tooth when driving can place you on your ear to put it mildly.