You're correct, but...
Larry is correct that sniffers can't tolerate contact with oil.. it contaminates the sensor module in the sniffer.
On the other hand, the manual on my MAC says the module goes bad all by itself over time ... and they recommend replacing it every year anyway. They come two to a pack and are inexpensive... takes about 60 seconds to change it. On my MAC, its mounted on the end of the "wand"... and there is a shield over it to keep it away from oil contact.
As far as eBay is concerned, I've found that most reputable eBay sellers will guarantee what you buy isn't DOA.. if you write and agree to that before your bid. Sniffers are a piece of electronics, so you need to agree to some sort of guarantee anyway before purchase. If the answer is no, buy it from someone else.
For fittings/seals under the hood, if you have a dye injector, UV works great. I've got specialized dyes (different colors) for refrigerant, motor oil, hydraulic oil, coolant, P/S fluid, etc... all different colors under UV light. I've found that UV dye is absolutely the best way to find a coolant leak... like those nasty ones that you can smell but can't find... once you have a UV light, the dyes are very inexpensive... and a little goes a long way.
Good luck. KenP