Why not sell at auction?
1) It's a consignment, not a sale, and you don't control the pricing. You can set a reserve for a desirable item. In that way it's like many consignment platforms, except that it's not immediate.
2) Beyond your low reserve (which has to be at or below the low estimate), you cannot control what the item sells for. Could be really low, could be really high, but you are waiting til the auction (usually months) to find out.
3) Most houses take 10-15% of the hammer price for your consignment as well as their selling premium. So remember, when you see in auction results that a bag sold for 20000€, that means the hammer price (with a 25% premium) was 15000€ and the consignor will be getting around 13500€ or less. So, if you could get 20K€ for your item on another platform and the consignor took 25% or less, you would be better off.
4) Unless they really really want your item, many houses will also charge you an "illustration fee" for photographing your item and putting it in their catalog, as well as insurance while it's in their care. This isn't much, but still a few hundred dollars.
5) Finally as noted above if your item is VERY successful the house may also take a performance rate.
It's a very valid way to sell (I sell a lot of art at auction although not bags), but you just need to understand all the fine print. In my opinion for bags specifically the resale market is pretty sophisticated and not subject to the same frenzy you see with art, so you don't have as much upside at auction. What you ideally want is a ton of competition for your item - that is what will drive the price up - so for something extremely rare when you don't need to get paid quickly, it can be a good way to realize a high price.