Depends on the situation and what kind of weapons RNG blesses you with. It also depends on how deep your pockets are, some of those weapons aren't cheap, and they sell for pennies on the dollar when sold back, so most of that price is simply lost when you upgrade again. It also depends on your skill in running caves. A skilled player can work around a bad weapon, but it takes a lot of knowledge of some less understood mechanics and knowing what rhythm you need to keep enemies at bay with your given weapon instead of simply spamming your attack button.Dang I haven't gotten any of these cool weapons haha. I have a black metal bat now~
Questions: Is it worth buying the weapons from the Adventures Guild? Floor 100 of the mines wrecked me and I lost a BUNCH of stuff haha.
For your specific case? Not really. At Floor 90, you got the Obsidian Edge, which is the third best sword in the entire game, behind the Lava Katana (which doesn't unlock until you hit 120) and the Galaxy Sword, which requires a Prismatic shard. So at mining level 100, there's no better weapon you can really be equipped with, really.
Your problem isn't offense, it's defense. The deep mines (80+) really take the gloves off. Shadow people and those armored dudes are brutal in melee, the armored guys have resistance to knockback as well. The shadow shamans debuff increases the damage you take, and those fireball spitting dudes can also be painful. Even the red bats found at these levels do way more damage than the icy bats of the previous tier of floors, which combines to make for a very lethal area to wander around in. So your concern isn't offense, it's defense.
However, as of 1.4, there was a bug introduced that prevents defense values from weapons (specifically and exclusively weapons) from functioning as intended. So while there are weapons, such as the Lava Katana, which have a listed defense bonus, they currently don't work, at least not until it gets patched (hopefully in 1.5). This leaves you with your boots, consumables, and the Crabclaw Ring. Head back up to a higher level and start grinding Rock Crabs. Kill 60 and the adventurer's guild awards you with a ring that gives you +5 Defense. Quite handy if you're having trouble surviving.
Alternately, bring food (such as salmonberries or cheese or salads) and be cautious with your progression. Don't just dive into the middle of everything, plot your path carefully, use terrain to your advantage, and don't be afraid to hang back and chow down to restore health if you start getting low.