In my opinion one should NEVER leave neutral or negative feedback before at least contacting the seller and giving them the opportunity to rectify the situation.
It could have been an honest mistake. We're all human.
I've got over 3,500 feedback on
ebay, split roughly 60% as buyer, 40% as seller. Been eBaying since 1998. I've been on both sides of this equation.
There's always the possibility that the seller may have deliberately misdecribed the item, but as a seller, there's far more downside than upside to doing this, especially on a $20 sale. It's just not worth it.
As a buyer, I don't want to burn bridges by preemptively leaving nonpositive feedback without any attempt to communicate. It sets the wrong tone RIGHT out of the gate.
If I have problems with the condition of merchandise, I contact the seller and explain what the problem is, and if possible I provide high-resolution pictures highlighting the issue(s). A picture is worth a thousand words, and also helps to prove to the seller that it's not just a buyer trying to pull a fast one (yes, there are plenty of those out there). Also, and this is important: I am polite... not demanding or confrontational.
The seller may come back with any of the following responses:
1. No reply at all (worst case scenario; then you deal with it through eBay/PayPal)
2. Seller disagrees and/or doesn't want to allow return (see above)
3. Seller allows return
4. Seller offers partial refund for you to keep the item
5. Seller says keep the item and gives you a full refund
Ok, granted, #5 is unlikely, but I've had all of the above happen, and #4 more often than you would think. It's less of a hassle for a seller to do a partial refund and be done with an item than to have to wait for a return, do a refund, relist it, and wait for it to sell again.
Again though, the response you are likely to get from a seller depends largely on how you as the buyer set the tone... and firing the first shot with nonpositive feedback is not the best way to achieve the optimal result, at least in my opinion.
Take the time to talk first... you've got 60 or 90 days to leave feedback.