I think shop rules shouldn't be too difficult to work out...
let see firstly, I don't think Profession skill is relevant for running a shop (i.e being an employer) as it is more pertinent for employees of someone else.
So where to start..
CONCEPT
What kind of shop is it... lets say an Inn you could use various sytems for building the Inn like the Stronghold Builders Guidebook and so forth...
SERVICES
But for a mechanics standpoint lets assume you have your site for the Inn, I would say first you need to look at what services are being offered, with an Inn i'd list the following.
1. Drinking Customer
2. Food
3. Boarding Customers
4. Stabling
These things are your sources of income.
PATRONAGE
Next you need to consider how many people will be using these services each open period, this is likely to be tied to the size, capacity, location and quality as well as the reputation of your facility.
So lets assume the standard inn has a capacity for 50 people in the bar room, 20 of whom could be seated and hence feeding customers, and has a capacity to sleep 30 people 1o of whom in private rooms the remaining 20 in a common room.
Base Drinking Customers 3d10 /24 Hours
-2d10 if trouble previous night
+1d10 if some form of entertainment provided (+2d10 if renowned performer)
+1d10 if in a low class district
-1d10 if in a high class district
+1d10 if Inn has a positive reputation
-1d4x10 if Inn has a bad reputation
Base Feeding Customers 2d10 / 24 hours
Modifiers as above plus
-1d10 if Inn has an obvious Vermin problem or other health issue
Base Boarding Customers 1d10 (rooming) 3d10+10 (common room) / 24 Hours
Modifiers as Above plus
+1d10 if Inn is safe and rooms have locks
-2d10 if Inn has been robbed in last 24 hours.
-1d10 if Inn has an obvious Vermin problem
+1d10 if it is Winter
Base Stabling Customers 5d4x10% / 24 of hours of total customers have mounts which need stabling.
PRICES
Then all you need to do is consider your prices for the Inn if you use standard PHB prices
Room (Private) 5 sp / day
Room (Common) 1 sp / Day
Ale (Mug) 4 cp
Common Meal 3 sp
Pitcher of Wine 2 sp
Stabling 5 sp / Day
INCOME
If you assume every customer eating has a pitcher of Wine, and every drinking customer has 3 mugs of ale each per night, this will allow you to work out your average income per person.
e.g
Each Drinking customer spends 12cp
Each Eating customer spends 5 sp
Each stabling customer spends 5 sp in addition to any other costs they may incur
And Each Rooming Customer spends 5 sp (Private Room) 1 sp (Common Room)
Of course these are average prices you could vary patronage each night by +1d10 for every 10% of price below average, or -1d10 for every 10% over average price if you want a greater or lesser turn over based on price.
Then consider 1d3% of the total income for a night is lost because people skip thier bills.
and 1d3% of the total amount each night is gained extra due to tips, this should allow you to work out the total income for an evening.
EXPENDITURE
Once you have the total income for a night worked out you need to work out your total expenditure in order to work out your profit.
First Wages:
Stable Boy/Groom 15cp/day
Cook 1 sp/day
Waitress 1 sp/day
Bouncer 2 sp/day
Entertainer (optional) 4 sp/day
Running Costs:
1. Rent if any...
2. Protection money to Thieves guild if any...
3. Bribes to Town Guard in event of trouble if any...
4. Taxes 0.1% of total nightly income (meaning you pay on avergae about 3% total income in any month in Tax)
5. Supplies if you consider that there are 395 pints in every 50 gallon barrel of ale (that is roughly 100 peoples worth of drink per barrel) and each barrel costs 10gp.
ALSO 198 pints in every 25 gallon cask of Wine (or roughly 40 eating customers per casks worth of wine) and each cask costs 5gp
ALSO Each meal costing roughly 15 cp in materials
6. General Maintenance, this covers repairs, replacement furniture and the likes and is 1d2% of the sites total value annually.
7. Advertisement (Optional) for every 50gp spent on advertisement each month increase the total monthly profit by 1d4%. No advertisement at all, simply relying on word of mouth will mean the business suffers a 1d6% loss in total profit over the first 12 months of business until it becomes established.
And you should then be able to determine how to run a functional Inn.. there is some paperwork involved obviously and the above steps can easily be modified as appropriate based on the kind fo shop or business you wish to run.
hope this helps?