That's the thing. There are really all sorts of situations and solutions.
There are company sales reps, usually referred to as salesmen or salespeople. The card may say rep, but not quite the same thing.
Then there are what are generally called "manufacturers' reps" or "manufacturers' agents". These are sometimes individuals, but more often comapanies who represent or sell for a variety of companies. They may sell your tee shirts, someone elses blouses, someone elses blue jeans, etc. They may also rep for some accessory lines and maybe even display fixtures and other related products.
By using a rep organization you can have a much broader footprint than you will with a single salesperson. Many rep organizations cover multiple states, or entire regions. They may also sell to retailers you may not even consider prospects. You could end up with your tees in a car wash waiting area.
The thing is there are no hard and fast rules. What works best for you?
Years ago I worked for a company selling hardware that had 6 price levels, with commissions as high as 40%, down to 5%. The 5% was for approved "national accounts", the rest was entirely my option. And I could sell different products off different price schedules. Sort of whatever the traffic will bear.
Think outside the box!
A museum gift shop is going to sell the same item for more than a flea market. Why shouldn't you make a bit more off the gift shop? Share the wealth!
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