buying second hand lenses from ebay, yay or nay?

stupid question with certainly a long history of answers but i have to! i don;t see pinned threads here

buying second hand lenses from ebay, yay or nay? I got the Nikkor AF-S DX 18-140mm f/3,5-5,6 G ED VR for my d5300

from ebay for $145, when refurbished ones are seen at $220 on amazon, and i thought i made a good deal.

how do i know?

the more i look in fine details in all my pictures, it looks blurry. i thought this was the "sharpest" one?

how do i know what i am supposed to have if never put my hands on a brand new one?

how do i know if it has an issue? is there a way to test?

I have bought used lenses and other photo equipment on e-bay, but from either very local people, or people with a 100% or close to positive rating with success, and only ever had to send one item back, (that's where the 100% rating comes in to play, and I would buy from her again).    I do prefer however to buy from established mortar and brick photo dealers, such as UPP, KEH, MPB, Adorama, or B&H as they all offer easy returns, a phone number where I can talk to someone, and a warranty.  Also, I've dealt with these established dealers and each of them have a very conservative grading system where you actually get a better piece of gear than you're led to believe you're getting.  I've sometimes wondered, "Why grade this as only, ''good''?  I see no defect at all, it's like new!''   Last year I bought my second Nikon D700 from UPP for the price of a nice used D5500.  It came with it's original box and everything in it as new, but with just over 8,000 shutter clicks.  They included an extended warranty of one year at no charge, plus 1% in bonus points toward my next purchase and a card for 5% toward my next purchase.  I used my points and card to buy a D750 saving around $80.00 on it.   Stick with these good established dealers and you won't have a problem, they list any defects, and sell defects as, ''for parts or repair'' mostly.  Some lenses will list as, ''good-with dust and mold'' avoid these and buy better.  They're honest about it. 

At times I've found that if you put an item on your ''watch list'' on e-bay, sometimes the seller will contact you with a better offer, (sometimes not).  There are also those who will accept a reasonable offer from you.  Educate yourself on what you want to buy before you shop, compare prices, and make your choice.  Sometimes that means waiting, and sometimes you miss out, but there's always another one.

There are dealers to avoid which I won't list by name, simply put, if they offer stuff at a price too good to be true, or only offer a store warranty on new stuff...

That's my view and experience on buying used photo gear.

gotcha

also true, i noticed this year that ebay enabled this new feature "contact watcher for an offer" and used it twice

i understand the mold/fungus stuff, but to your knowledge, what does means "tiny dust" in a description? dust inside the lens or outside?

That would be inside the lens, a few tiny dust particles shouldn't matter as to picture quality, you want to stay clear of a lens with mold though.  For photo gear I deal with 99.8% positive sellers or better.  Make sure you can return what you bought hassle free, it's also a good idea to take a picture of the listing before you buy, in a dispute e-bay will most likely side with the seller since that's where they get their fees from.  If the seller is a brick and mortar store, they have it in their store for less since they can forgo e-bay's fee.  Google them and shop direct if you can.  I just looked at a lens today from a dealer I know and deal with, their store price was almost $20.00 less than the e-bay price, and free shipping, plus earning around $3.75 in bonus points I can use later on my account. 

Camera bodies come and go, good glass will out last several bodies.

When buying, try to practice the old saying, ''Only cry once.''