I've leveled two merchants up to at or around job level 50 recently. A lot of the ideas people are giving you are good ones, and personally I tried the poison spores ideas but it didn't work out too well as far as resource consumption goes. It could be that I suck at mobbing 'cause I'm really not the best player. What I used was metalings. Here's what I suggest:
*give yourself just enough dex to reliably hit them (for me like 16-18 was enough), and put everything else in str and vit, about evenly but if you have to make a choice make str higher. The extra vit will reduce the amount of healing items you'll need because you'll heal more efficiently, and have a higher HP total too.
*If you can get one for cheap, get a "Cutlass." It lets you use the skill "Bash" at lv 5 for like 8 SP. I randomly got mine out of an old blue box, so I don't know a good place to actually get them, it's just an easier, less costly alternative to Mammonite. If you can't get a cutlass, use Mammonite. Don't use higher than lv 3-4 on it even if you decide to put some more skill points in there. It wastes too much money at higher levels, but if you set your status as I suggested, you should be able to get away with killing metalings using 3 mammonites back to back. Later on, you'll be able to do it with 2 and a single normal attack. Learn how much HP they have and known when to switch to a normal attack and you're pretty much set.
Another thing... procellios give not very much more exp than metalings, but every now and then it's a good idea to kill them because they pick up your drops and you want those drops to make money that you will be spending on meat, GJ, and mammonite.
Changing my two merchants into an alchemist and a blacksmith I did also happen to get 2 metaling cards. I sold the first for 9.5 million and the second I gave up on trying to sell because people don't seem to want them anymore.
On the other hand, in the short time I tried poison spores, I got at least two karvos, so you can make some money off that too if it's money you want/need. And cart revolution doesn't cost you money to use.
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