I've used Ebay for buying some vape stuff. I always check the sellers history of course. If they've been selling vape gear for over a year and have tons of positive feedback...little negative...etc...I'll risk it.
First, I knew it could be had for %50 less direct from Hong Kong; but, I needed it quickly and did not feel like the hassle and wait time of over-seas transactions. There are also times when ordering direct from Hong Kong requires bulk orders...or it's not 'that much' of a discount.
When it's a $30 item with free shipping...I just have to ask myself if $15 is worth a 1 - 3 month wait...by the time I got it local vendors would probably stock it as well...and at similar mark-ups as the Ebay price to boot.
Also, there are a few things that Ebay sellers manage to get a few weeks (or even months) earlier than the better known vendors on the Grid. Sometimes it's because vendors are waiting to clear out older inventory...or maybe they're still in testing phases before 'stocking up', or...they have no plans to 'carry' the products at all.
Heck, it's even possible that some of the 'vape shops' do run a test phase on Ebay before 'stocking/listing' stuff under their brand name. See how it goes that way before trying to 'back it' under the main store name.