The Lineage II auction house is outdated compared to every other MMORPG out on the market.
It was a great step forward a few years ago that we didn't had to do private stores any more to sell everything we had, or everything we wanted but the Auction House lost its valuable purpose.
The auction house has to be overhauled and improved.
Current issues:
You cannot search for a specific enchant level. If I want to sell my +10 Krishna Thrower I have to first filter on R110 Weapons and than filter again on Bows. The search box doesn't allow to search for +10 Krishna to filter out all +10 Krishna weapons to do value/price comparison.
The search engine is rubbish and doesn't allow searching for specific item names. Let's say I want to search auction house Chef Monkey's Belt. An older item but still. The search will give no result while 10 are in auction house. All because the search engine doesn't understand special characters like the ' which, sadly enough, is in almost every single new item in the game like for example Angel's Ring, Dragon's Ring, Eva's Necklace. It makes searching for items a chore.
You cannot buy partial stacks. Thanks to all "events" market sometimes get flooded with items. Few months ago thousands of Rune Stones were flooded onto the market, more recent Gem Energy. All of them are being put into auction house in either stacks of 1, 5, 20 or just big bulks tacks of 100+. But if the lowest price in the auction house is a stack of 200, and I only want 4 I have to buy the whole stack of 200 OR buy more expensive smaller stack. Why every modern MMORPG allows you to buy 4 of that 200 stack, except Lineage II?
Majority of items cannot be put into auction house. Items like Cloaks, Circlets, Shirts, Bracelets all are locked out from being put into auction house. Every modern item basically has a version or restriction that cannot be listed. While obsolete junk in my warehouse from 2009 can happily be listed. Either allow is to put everything in auction house or nothing at all.
Make auction house accessible everywhere, not just via the NPC! You already have added the option to access the Dimensional Merchant from an on screen menu. The Dimensional Merchant really doesn't do much and for majority of the players is a feature they never need(ed) and never used. Adding direct access to the auction house from the same menu bar as Timed Zones, Ranking and the Dimensional Merchant allows us to buy and sell items from our inventory straight from the hunting zones.
Not an issue but maybe good for market
Add the 5 newest listed items on the blank screen of auction house. Why prompt us with a blank screen, if you can list 5 most recent listed items.
Add the 5 items with biggest price change. This is very common in modern MMORPGs. The auction house/central market/marketplace listing a bunch of items that recently had a big price change (up or down wards). This will allow everybody to find bargains (or wrongly listed items) which allow more fun market play. Usually all the wrongly listed items are picked up by the bot chars that stand near the auction house NPC and run their script every minute.