My tuppence worth . . . . .
To have an online presence selling property, you'll require a large amount of marketing budget to compete with what's already out there, even if it's local advertising.
For a pop-up shop, it works both ways. The landlord will want to include a 30 day (it will depend on each landlord) vacant notification to terminate the license, should he have an enquiry to sell a lease to another party. A quick exit is probably the last thing you need to build your reputation in the local area you're looking to start with. Even on a short term basis, you'll need to invest in getting the shop up to scratch, as well as furniture, phone lines, signage etc etc, presuming the property is in a good decorative order.
My advice is to commit to a year's license (rent to be negotiated in your favor - 20% of current rental value is probably fair on both sides), so at least you have security of tenure for one year with a clause to allow you to extend, should you want to.
Take into account that if you have a B&M shop, you'll have additional expenses, staff, utilities etc etc, which all needs to be taken into account.
Even if you decide to just have an online business, you'll still need premises to work from, whether it's an office, shop or other commercial property.
I agree that sellers will prefer to work with a local agent rather than an online only agent, but they'll probably be directed to an existing company they trust and has a reputation in the area, rather than a newcomer.
Your only way to get a seller on board is to offer a cheaper rate, probably 1%, instead of the normal 1.5% - 2%.
Give it some thought and see what competitors you have in the area you're looking to start with.
