In addition to what's already been mentioned, barrel length affects value with these guns; 2" is worth more than 4"*, and 3" is worth considerably more than 2". (*I'm not entirely certain that a 2" M10-8 was made, as I don't have my reference sources handy, but I believe that this version does exist.)
IMHO $400 is definitely on the high side for a 4" gun in the described condition; I'm with the other posters here, $300-$325 is the going rate for these. However, a 2" or particularly a 3" "shooter" could certainly go for $400 depending on local market conditions.
(As an added footnote, 5" and 6" Model 10's exist in tapered "pencil" barrel form, but AFAIK these lengths were not offered on the bull-barrel M10-8.)