Where to buy
* Buy from other photographic lover, they are mostly trustworthy people, they will tell the truth about lenses.
* Buy them personally, if it possible, in local photo stores or other market.
* Buy from the Internet, for example on Ebay, if you have no other solution and don't worry about that too much, I bought around 160 items from the Internet. I got only one or two times worse lenses than described. Pay with PayPal and order always insured, signed-for shipping.
How to check the lens before buy
Bring with yourself into the shop air-blower and flash light. Blow the dust on both side of lens with air-blower and light through from both end. You should see any problem in strong light. If the item is mint condition and high priced, you expect crystal clear lenses without cleaning marks (light circular scratches), scratches, dust inside, fungus etc. Strong light show you all if there are any of them.
For lower price you can accept cleaning marks, small scratches, little dust inside because they don't impact optical quality at all, just make them hard to sell if you don't want to use them anymore.
For real bargain price you can accept almost every problems on glasses . Even if lenses are little damaged, it's optical quality is usually great, you can't see anything from the damages on the pictures!
I have more experience now if lenses with fungus stored right , under below 50% of humidity without caps to get some light , fungus not growing at all and not infect any other lenses!
Check carefully every function: focusing must be able to turn smoothly to the end and back, but depending on the price, hard focusing can be ok for cheap price.
Set the aperture at least into maximum and minimum position and look through the lens, to check is it working properly or not.
If the lens has it, look at the auto-manual switch, if you planning to use it on DSLR, you must use it in manual mode. In auto mode is the right working stage very important on analog camera body. Try it always with camera body if possible.
Buy from Internet:
If you can't test personally, you must trust in the seller. Ask the seller in advance every important thing. If it is not described, you also need to ask in advance the safe postage cost /insured, signed-for/.
Watch out carefully seller feedbacks and his return policy.
Don't buy from this kind of sellers:
"I have not much clue about photography, I just got from my friends" - this means: the item has a defect, mostly fungus or other serious problem.
"Sold as no return":the same than above, but not in every case. If you are enough brave to risk a few money sometimes you can make a good deal, sometimes you don't.
Anyway many good lens are available only on Ebay, so take some risk and you should get valuable pieces.
Thank you for taking time to read my guide! Please visit my Ebay shop to get some lenses :) MFlenses