1. Caveat emptor. Ideally, buyers should be aware that, if the product doesn't carry these certs, it may not be safe to operate for long, or in general. (Safe in regards to, say -- knocking out nearby emergency services, would be the worst-case sort of condition.) And therefore will make purchasing decisions accordingly. The reality is, ain't no one got time fo dat, price is king, and eBay is an unregulated marketplace.
2. Typically, the end user or importer is liable. Obviously, the manufacturer or distributor bears responsibility too, but I think that comes down to truth in marketing. Don't claim it's FCC approved when it's demonstrably not; don't sell it as a general-purpose product when it's clearly a component, or test equipment, or whatever.
3. Consider how violations are processed. A licensed user detects interference and submits a complaint; the complaint is investigated, and a source identified. The first letter is a C&D to the user -- stop operating the offending device. If they continue, the process escalates. If the device is approved but found to be faulty, it comes back to whatever the source is; if the device was imported, it may dead-end at the importer, since law doesn't usually extend beyond borders (but, depends on treaty).
At least, I think that's a roughly accurate overview of the process here. YMMV, IANAL.
4. Also along the gray line of the law -- reports seem to be quite rare. I know of products that were distributed in the 10k's quantity that never went through FCC, and that apparently never got a call about it. Testing is costly (figure ballpark $10k with a testing lab), and it doesn't make sense to do for small quantities.
You want to air on the side of caution, but it is absolutely just another business gamble. If you're willing to take that risk, it may pay off. (Reminder that legal fees and fines are accounted for, and written off, just the same as any other business expense, even to the IRS themselves.)
5. Speaking of test equipment: it is typically exempted. Since, a lot of TE intentionally makes nasty fields, at least on the hot end, so, what else could you do? The catch is, it must only be operated in a shielded room/facility, by authorized technicians. Which... who are you to judge? There is no state authorization of technicians, it's up to the buyer/end user to determine that. So, the manufacturer is not liable in this case, and it's up to the end user to follow procedure. (Again, "I think", IANAL and all that.)
There are also exemptions for distributing articles intended for testing and not individual sale, but that I think is basically "dotting the i's and crossing the t's", not actually useful for marketing...
6. FCC doesn't require immunity testing (they do at least suggest it), but CE does. YMMV.
Tim