Does not void warranty. You just have to put the original cooler back when sending the card in for RMA.
See: https://www.evga.com/support/faq/FAQdetails.aspx?f=58128
Why would it decrease resale value?
@unknownexception made it the XC3 Ultra can be made to fit.
https://smallformfactor.net/forum/threads/dan-a4-sfx.252/page-181#post-220190 see this and the next few posts.