How would I play a merchant pc?

[MonkeySage]

The most recent project of mine is to create a new character for a friend's solo campaign; the character is a halfling sorcerer/silver dragon disciple, and a seabound merchant.

So, without getting a masters in economics or business, any tips for playing as a merchant?

[Gildedragon]

Buy cheap, sell dear.
Buy in bulk and variety.
Haggle! There's rules in dragon magazine for that sort of thing.
Craft your own stuff. You won't be able to beat an artificer at that (the best merchant class).

Since it is a solo campaign then tailor the adventures around commerce. Introduce money-changing mechanics if one is so bold.
Quests to find a particular source of an exotic good. Quest to reopen or secure a trade road...

[Inevitability]

Depending on how morally ambiguous you're willing to get, an Extended Fool's Gold can greatly increase your profits.

You could also take Arcane Disciple (Trade) if you're wisdom is high enough, netting you neat spells such as Gembomb, Fabricate, and even Mind Blank later on.

[Kraken]

DMG2 has rules for operating a business. Be advised that they've got some pretty significant faults, so you might ask your DM to go over them and make tweaks. You could also look to Pathfinder's kingdom making rules for some guidance, their purpose is a bit different obviously, but many of the principles are similar.

[MonkeySage]

Think I've got some ideas going... Oh, anything I should tell my GM? This is kind of new territory for him as well.

[Gildedragon]

Get him to watch "Spice and Wolf"

[Troacctid]

Consider the Merchant Prince prestige class, from Power of Faerun. It gives you a great boost to your profits from running a business, and offers a nice selection of spells that are very handy for someone working retail.

Edit: Although if you're already a sorcerer, it is somewhat less useful, I suppose.

[Tvtyrant]

Buy lots of extra dimensional storage, take everything you find, constantly seek after trade opportunities, invest in other businesses. Ask for shares in a town's businesses instead of lump sums for instance, make weird get rich quick schemes like perpetual motion devices and farm-boxes (big extra dimensional space with Hallow cast in it with Daylight spell attached. You now have invented the perfect drug growing apparatus).

[Teh_das]

There is a player in one of my games that recently took Leadership with the plan of opening a shop. He's the owner and adventurer finding the wares, his cohort is the shopkeeper.

[Theobod]

There is a player in one of my games that recently took Leadership with the plan of opening a shop. He's the owner and adventurer finding the wares, his cohort is the shopkeeper.

Having done this sort of thing, and avoiding messing too much with magic to achieve it, some nuggets:
Loot everything and hold on to it, sometimes you will get further trading goods for services than using pure currency as the value of these items may be more than the service is worth but equally sell for less than the service is worth (example: Want a 75GP horse, give the man a chain shirt to protect himself from horse rustlers worth 100gp, this is better for you as you only sell it for 50gp)
Ask the DM if its possible to sell for more than half price on items assuming you set up a stall, I have had a DM periodically look over my goods once per week and randomly select what sold (usually a handful of things if I set the price to 75% or one item if it was set to 100%, never be afraid to undercut)
Don't hang around in one place very long, turn up, set up shop, leave an npc there selling for you (hireling, cohort, follower whatever), go and buy whatever you need, maybe pick up a quest, then leave. This prevents local merchants getting too irate about their profit dip from your undercuts and means you don't saturate a market, thus dropping your own sales, a couple weeks tops per place.
Don't be miffed if you don't sell much each trip, if you looted it for free anything you got is profit, trading is basically a mini game to try and drain a bit more gold out of your WBL, it doesn't always work and local circumstances wont always lead to a conducive trade environment, you should be on base WBL anyway due to adventures so treat mercantile practices accordingly.
Craft! Even if you have to sell items at half worth because your DM refuses to allow anything higher despite the NPCs selling it to you at full price, every single time without explanation (or worse: Just cos thats the rules), you can still make money as a trader. Buy raw materials for 1/3rd of the value, sell completed product for 1/2. If you are so inclined add in certain spells to increase check result or base craft time to maximise this (the DM may rule that some places wont buy more than a few units of what you are selling due to supply n demand, so keep a varied selection and, if the DM has had recourse to this argument, turn it around and talk about undercutting tradesmen as above.)
And importantly, even if this is the entire reason the party is adventuring, dont let it hog too much spotlight time, chat with your DM about how it will work and try to make your trips into town speedy, know what you are buying beforehand, have a list of things you are leaving your trusted NPC to sell ready to hand the DM for him to tick off appropriately and then get on with the adventure :)

IF all of the above fails, just max Profession: Merchant :) If you travel from port town A to port town B and it takes ten days, that's ten days of profession checks, you get the payment for on arrival. Ask the DM if a ship gives a bonus (and if its bigger if you have a cargo vessel and take up the whole hold all the time) and if having a varied stock also gives a bonus, not to mention if the social skills synergy, get party members to sink in one rank and aid another, same for all your NPCs, you should be able to take ten for 30+ easy peasy.