three common principles: detail (quality of materials and craftsmanship), modestly (covering skin), shape (cupcake, puffy a-line)
a dress is the main item and saving too much here can be more visible. a petticoat is required for dresses
for ouji style, the jacket seems to be what a skirt is to other lolita styles
building a wardrobe or a coord can be a challenging, long term and expensive process
costs
for a typical high quality outfit i would say 500-1000 euro (dress 200-400, blouse 100-200, shoes 50-150, other 100-300)
some veterans have $20k or more invested in their wardrobe but this is more if you want to wear something different frequently
some basic items are currently hard to get in high quality (wristcuffs, socks, blouses, shoes, etc)
storage: it can make sense to sort clothes by season, coord or style. otherwise sorting by top/bottom/socks/headwear/other is usually a good way
second hand items are items that people dont want to have anymore. this can be because the clothes arent fitting size wise,they prefer a specific look that doesnt require the items, they are wearing less items of the fashion in general or it can be items that are more difficult to coord or have features that make them less popular. good items are naturally quickly bought - consequently what is left in shops always requires particular scrutiny and research, to not overlook some detail you wont like
blouses
for blouses it is about what you can see when you wear them. collar and sleeves are the main points to look for. material (plain, dotted, striped) also has an impact
cutsews can be used like blouses
brand items
have a decent resale value. i observed these rough value changes: new to used - 50% reduction, used to used - depends on popularity of item
tend to be quickly sold out and the best items are sparingly available second hand. items might only be available in sizes too small. a speculation about general availability: some people might buy items and not resell them since there is a big collectors aspect to the fashion, or some items are passed between members of closed off groups
other benefits are longevity, detail and craftsmanship, thicker garment with less see through, better thought out cuts and design