Having caps and rings (if in decent condition) definitely help some, but a mismatched set of 6s and 5.5s is obviously a bit of a drawback. Date codes and part numbers also do affect the value, for example if the 5.5 is a 1970-71 wheel someone will want it and pay more for it than a similar wheel with say a 73 part number and date code.
I recently sold 2 sets of bare rallye wheels on craigslist, they were all sandblasted and primered, the set of 5.5s went first for $100, and the guy came from a fair distance to get them. I let the set of 6s go for $80 because I wanted them gone. These were all 72-74 part numbers IIRC.