You asked for "tips". I partnered to have owned 3 hi-fi stores getting out in 2001 after 30 yrs of doing it.
Although your store is simpler in product, the thing that you must watch for is "expense creep". You will own the business. Are also owning the building or is he?
Rent can go up, Business regulations met such as fire code and equity clauses. You may be strapped to install the latest fire suppression or widen doorways for handicap access that the owner may apply to you in some fashion.
Differentiated tax codes. State regulations as they apply to transfer of ownership clauses.
New regulations as they apply to sq footage of stores.
10K seems fair to me but I would stipulate payment on a royalty basis to prove how valuable his customer base is within the area itself. If he is "re-opening" somewhere else, he is basically taking his customer base with him if most of his sales are on-line.
I may have missed some stuff but you get the idea.
The killer is in the details....and "customer count".