I think you are looking at a Single-Pilot or Single PIC operation in the FAA eyes.
Buying any Part 135 certificate is not an easy transfer. You should run it by your FSDO. Most likely they will have you do or redo all the steps to get a Single-Pilot or Single PIC Air Carrier Certificate.
If the seller is placing a high dollar amount on the certificate it may not be a good buy for you.
The FAA is picky on the aircraft. It needs to know that the aircraft is in annual, 100 hour, all AD’s and Service Bulletins need to be complied with. They may need all the serial numbers from all components like mags, ELT, etc. even if the aircraft has been in use by a Part 135 operation. The do an in person conformity check where they look the aircraft over for any visible squawks. Missing screens, cracked interior plastic items can raise red flags. If the interior has been redone you will need the burn-certificates for the new interior fabrics etc. Purchasers get burned (no pun) when they buy an aircraft for Part 135 and a car shop has redone the interior and no one has any burn certification documents. Engine times must be below TBO or 12 years whichever comes first. Most mfgs use 12 years but check it out.
There is basically NO Part 135 instrument flying in SE aircraft with PAX perhaps except for C-208 and Pilatus. You need DME or IFR GPS if you do instrument flying under Part 135.
Single Pilot means single pilot who is named on the OPSpecs.
Single PIC can have I believe up to 3 SIC’s ( a training manual may be needed)
Both of the operations listed above need Letters of compliance for Part 119, 135, some of Part 43.
A letter of compliance is basically rewriting each reg and stated how you will comply with it.
An MEL is good to have. Ex. if a NAV light is out you can’t even fly in the daytime without an MEL. An MEL will state what you can and cannot do. If the Transponder is out you will need a lot of verbiage explaining in the MEL what you can do.
I have done the paperwork for several small operators. PM me if you have questions.