Best to be done a couple playthroughs in, after you've unlocked all the dungeons and have a L100 adventurer in full Angel gear or better. Make sure you have high quality Racks and a fully upgraded shop for higher Style, as well as having a high Prestige to attract more customers each sale period.
It has to be done in one playthrough. Make several runthroughs of the 9th dungeon, the Ruins, to gather L90 Angel equipment. Each piece sells for 10,000-35,000 gold each without accounting for haggling. An alternate source of income is to buy and sell/vendor Demon Armour from the Guild in your shop. (8084 gold each to buy, but a base sell price of 11,550 gold for a profit of at least 3466 each.) It'll take a fair investment but it will pay for itself after a few days of selling.
If you're STILL lacking a little gold and you're right near the 60th day time limit, remember that you can sell racks, vendors, monster materials and any other spare stuff still remaining at the Guild. None of that carries over into later playthroughs anyway, so just sell the lot of it on the final day. It might just make the difference.