I've found it to be worthwhile so long as you stick to a general strategy. I go to a lot of thrift stores/flea markets/etc. I figured since I'm going to go to them anyway looking for cheap alternatives to retail products I need/want, I might as well keep an eye out for things that I can turn a profit on. The more I've done it the more I've refined my strategy and found out some things:
1) I tend to stick to vintage/unique items. Reasons: because they are vintage/unique people tend to expect or not notice flaws on these items, because it is understood they are secondhand and old...so I get few requests for returns/refunds. I just try to be as honest as possible on the listing. Despite being vintage there tends to be enough of a sales history on ebay on similar items that it's easy to copy similar listings (other people have already done the research). At the same time, they tend to not be common enough that if that right person stumbles across your listing they'll buy it.
2) Take care listing anything related to electronics, as well as popular collectibles. That's where the scammers seem to focus. More than anything else its other ebay sellers using disposable ebay accounts to 'purchase' or otherwise bid up (and out) the competition. You're facing a lot of sellers who do this for a living and at least a few of them are not above such tactics. Go into your account settings and you can block accounts with (0) feedback from bidding on your items, doesn't stop them all but it helps.
3) Do your best to streamline/automate/reduce costs. For example, as soon as I get an item I want to sell on ebay I take photos of it on my phone and it immediately forwards them to my email. I set a side a few hours post-yard sales to research, list, and allocate a box/padded envelope for the item. Whenever a bid is placed (if using an auction) I go ahead and pack up the item. I bought printer labels in bulk (for roughly $0.02 per label) so that I don't have to print/cut/tape shipping labels. I've bought most of my supplies from ebay itself (scale, labels, packing tape - also in bulk). You should be able to find/get plenty of boxes for free (re-use Amazon boxes, Priority boxes from the USPS if your shipping priority anyway). The most annoying thing for me is trying to find enough packing fill (bubblewrap, peanuts, etc.) for free or nearly free. Try not to spend too much of your valuable time on any of these activities. I've recently begun incorporating a $1 shipping charge on all my items to make sure I'm getting my shipping supplies paid for by the buyer - it's small enough that I haven't had any complaints yet.
4) If you get a message stating that the item is broken, immediately ask for them to send a picture (ebay makes it easy to attach a photo to internal messages) to verify. If you get a message stating that the item is missing pieces/components/etc. upon their receiving it and they are demanding a refund, ask if they would be willing to return the "defective" item. It's perfectly reasonable for you to request even a "defective" item back before sending them a refund. In my experience this eliminates more than half of the refund requests (I state I don't accept returns btw), as I assume they are trying to scam me. If I do get a picture of a broken item, or a return of a defective item, I refund them. The point is to weed out scammers.
5) Smalls are your friend. Whenever possible, look for profitable items that are physically small. Small items just make everything easier.
6) If you sell something very expensive (with a huge profit to you), you may want to cough up the money to ship UPS. The majority of the time USPS works but if they lose something, even if its insured, they don't give a flying **** about you or the buyers insurance claim. The buyer isn't going to bother with that and will demand a refund. So far it's been far easier to just eat it and give the refund.
I know ebay does seem to be going down hill for small sellers. Fees are increasing and ebay tends to assume the seller is guilty of crimes against humanity, but so far, as a side hustle, it has worked out. I have not given Amazon a try yet for selling but plan to soon. I think the key is just to find ways to reduce your time commitment to selling, so as to maximize your $/hr and making the hassle worth your while.
For anyone who sources from yard sales/flea markets like I do, one additional thing I do is also buy items that I can flip at more upscale flea markets but are otherwise not good enough (low margin) for ebay. Ex: buy an interesting picture/art piece for $1 and sell for $5. Once I get a shed full of these items I go to a local big/huge flea market on a Saturday morning and sell nearly everything. By my rough estimates it pays for my COGS.
*Edited to add that anything you don't sell or turns out to be crap - just donate to Goodwill and take the writeoff.
