I don't know anything about consumer law there but if you can't force them to accept responsibility (as discussed) then good luck with testing new memory with this board. Make sure you specify it must be compatible with your board and the safest way to go would be to get the seller to install and test it as well.
INTEL DG41 WV will not support Core 2 Duo 2.4 GHZ E4600 with 1333 MHz Ram, this is what an Intel Technician told me on phone call when i contacted intel regarding the memory compatibility of intel dg41wv? Intel Tecnician told me that this board will not support Core 2 Duo processor 2.4 GHz E4600 with 1333 MHz by applying some multiplicative factor, unless it is clearly written on the intel dg41wv box that "intel dg41wv supports 1333Mhz ram by downclocking it", Is this information is right?
You can see for yourself here: Intel® Processors and Boards Compatibility Tool - Search Results Page shows that CPU is supported by the board but note it has an 800 MHz bus speed so what the tech mentioned about the ram is correct.
Do you have any reason to doubt what they have told you? Just because something works for a while doesn't mean it'll keep working (or even not cause damage in some cases) if it's not to spec.