a lot of ppl frown on collecting for sale. however, if you only take every tenth or twentieth millipede you find there would probably be almost no impact on the area. and if you priced them to sell them as breeding groups (i.e. 1 for $7 or 5 for $15 you could actually help to get another species established in the hobby
when you start pilfering EVERY oNE you start getting into pretty dodgy territory... but it is likely if millipedes are in such numbers that you are practically stepping on them then you probably won't really be able to collect enough to seriously depress and area
but... some millipedes take years to mature and so are somewhat slower to bounce back from having a lot taken... so it is really pretty hard to tell exactly what the affects of collecting in big numbers is.
also, you could try colleccting only a few from any given acre and spreading your collecting out to hopefully only take a few from any given area
p.s. you might get some slightly flamey responses... try to understand where they are coming from and not take it too personally
plus, you are doing a bit of research BEFORE you do it, which speaks well of you
also, do you want the money to buy bugs? if so, you could consider trading them. you generally get better like, value when you trade someone, too