It won’t affect the trade-in value, Apple doesn’t look for ‘image retention‘ issues. The only thing that the watch has to be considered in terms of trade-in value, is fully functional with power on/off, and no physical damage.
Also, you won’t be able to ‘return your watch’ per se, but they may or may not replace it with a warranty watch depending how severe the ‘burn-in’ is, and if Apple can actually locate the image retention or the issue is minimally invasive, they will just send you watch right back to you with no warranty replacement. The choice is yours.