Well, I doubt that exp gain is of the utmost importance right now :p Right now he should be selling for as high as possible (115-125% x2 if there's a high price going). On the other hand, higher merchant level means access to better stock + vending machines (provided he can reach level 15 before running out of time to sell). They can be a life saver in getting rid of those hard-to-sell stuff. If that's out of reach because of time issues, then try the following:
- try keeping the atmosphere in the middle + a lil' scratch at gaudy. I noticed that the closer to the middle, the more customers show up in a time slice. And of course, you want to keep Alouette coming, since she can take lots of punishment in regards to prices (I can hog her for 260% when there's a high price). Prime's ok too, since despite being a greedy monkey (104% price at best), she usually goes for the expensive stuff and that in addition to high price still means a fair deal of profit. You can change the atmosphere depending on what kind of items you throw in front of the window (you'll have to experiment a lil) or from simply using different wallpapers/floors etc (which means more moneywasting sadly)
- since you already said you're reloading saves, try to keep the max efficient sales during the slices. Under no circumstances should you buy any high price items from the people visiting your shop (for obvious reasons). On second thought, try to get as little buying offers as possible and if, make sure they're cheap. If some items are in denial, pull them off the shelves.
- you could experiment with the max budget for all the shoppers. When asked to suggest an item, pick the most expensive one and see if they'll buy it at your regular price. If they won't, try cutting the price significantly (under 100% of course). That'll give you general idea how much you can squeeze out of the customers. Then just get as much stuff that's within their financial reach and sell it.
- keep the stock suitable to how much a customer is willing to spend. Don't invest into cheap stuff since in this case you need to rely on quality over quantity. Make sure you liquify all the cash into the items.
- Keep an eye out for item booms. As vinfang stated, you can get a real nice profit when you get a huge amount of people going for some stuff. In your situation the perfect would be a high price + boom on the treasures. Stocking up with golden scales for all the money could net you 300k provided all get sold
- Oh and yeah, did you expand your shop yet? It costs 10k pix if I remember correctly (don't take my word on it) but you should definitely take that, since you'll be able to place more stuff and either more room for the items to sell, or a spot for a vending machine to get yourself a bonus selling spot (you don't get anything more than 100% of item price but they're being sold outside the regular shopping queue so in addition to regular sales you'll always get those few more pix)
- The ultimate desperate attempt - Go dungeon crawling from the first floor and go for the last floor. I call it desperate because you'll have to farm the mobs and treasures for a long time and won't be able to afford getting KOed. However there's a chance that you'll leave the dungeon stuffed with some formidable valuables (especially the knights + bosses)
In general, if you can maintain a fair profit per slice (let's assume you'll spend all the slices selling, changing one slice once in a while to restock on supplies), it's most certainly possible to get the money. Hard as hell, but under favorable circumstances possible. Let us know if you'll be able to turn the tides