A vintage chemist bottle, you can find loads of these on ebay for not very much. Although they tend to be pretty sought after.
I have an odd love for microscopes, they tend to sell for a lot of money on ebay and are very sought after!
Beautifully coloured and well cut rock and mineral specimens always look good in specimen cabinets.
This particular specimen is Titanium Aura
Collections made by entomologists (hobby or not) tend to sell for hundreds, they can also be a pain in the ass to take care of. You should always keep insect specimens away from sunlight to avoid loss of colour. Good care and conservation of an insect collection can keep it around for years to come.
Preserved (or "pickled") specimens tend to be quite rare on ebay. Always avoid buying Taxidermy & Preserved specimens from foreign countries as most of it is illegal to import.
You can find many strange curiosities preserved in Lucite on ebay, most of it is legal to buy.
Whole skeletons are silly expensive and very fragile, but worth splashing out if you are serious about it and want to keep it forever.
Real Human skulls are obviously, very, very expensive. They also creep me out (yes, even I get creeped out).
You can sometimes find dried flowers, or fake flowers in little domes on ebay. This particular one is a sweet little brooch.
I was going to share lots of other goodies with you all but to be honest, I want them for myself!