Copyright law applies to copying. If you buy an item from eBay, and sell it on to someone else, no code has been copied.m by you. Therefore you have no obligations. Don’t make copies of the code in the device, because that can create obligations for you.
With GPL licensed code, depending on what the person creating copies and distributing them did, either the software was accompanied by source code (and the receiver has no further rights to receive the source code), or everybody has the right to a copy of the source code for a reasonable payment, with no need to buy anything.
Saying this again to address comments: As long as you don't make any copies, the GPL license doesn't affect you at all. (Also, if you only make copies that are allowed by your laws, then the GPL license doesn't affect you). If you buy then sell an item, without making any copies of any software, then GPL doesn't affect you. You can quote the license terms as much as you like, but the GPL is a license, and the license terms determine whether you are allowed to make copies or not. Nothing else.
Compare this to proprietary software: You are only allowed to make copies that your law allows, and usually you can't get permission to make more copies. Same as with GPL licensed software, if you buy an item and sell it again, without copying any software inside it, it doesn't matter what proprietary software is in there. You can buy and sell your smart phone for example, which contains tons and tons of proprietary and non-proprietary software.