I have done it, but you really need to look over the auction offerings carefully to find trustworthy sellers who sell legit Trading Pins.
There are so many 'fakes' and knockoff pins now you have to be very careful.
I have bought many pins off of eBay over the years, and several pin lots as well to use for trading in the Parks.
It can save you quite a bit of money buying in lots IF the pins are legitmate Trading Pins.
Most of what you will find in the legitimate multiple pin lot auctions are the smaller pins from various 'mystery box' sets sold at the Parks and Cast Lanyard pins.
These are fine for trading.
Some tips to help you buy wisely -
Only buy from sellers who have been on eBay for several years and have a 100% rating feedback.
This info is clearly visable on each auction listing for any item.
Also, only buy from a seller that has a return policy just in case.
If you get sent 'fake' pins or 'scrappers' ( official Disney pins, but 'unfinished' / 'seconds' ) you will want to get your money back.
When it comes to pin lots, try to buy from a seller who is showing you exactly what you will be getting so you can see what pins are included.
Versed collectors can immediately spot potential 'fakes', and folks just looking for a fun bunch of pins will see what they are paying for.
Avoid the 'grab bag' type auction offerings as eBay is full of them.
These typically offer a grouping of pins for a flat price but don't show you exactly what pins you are getting.
80% of the time, these 'grab bag' deals are filled with 'junk' and just a few decent pins.
Most also contain 'scrappers', the bane of the Trading Pin hobby.
Buyer beware!
Find a reliable seller, order a lot or two, and enjoy trading.
They are out there, just be careful you don't get tricked.
Happy trading!
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