Start with relevant experience, marketplace development company not the number of logos on the website. Ask for two or three engagements that match your stack, and then ask who actually wrote that code. A solid partner will introduce you to the tech lead. Evasive answers at this stage almost always mean you are talking to a reseller.
The contract deserves more scrutiny than the proposal. Three clauses do most of the work: intellectual property assignment, non-disclosure, and python development company termination and handover. Every artifact has to transfer to you as it is paid for, along with designs, scripts and retail ecommerce software development company infrastructure configuration. Look closely at any clause that keeps so-called reusable libraries outside the transfer, since this is frequently exactly the piece that locks you in.
Ask how they estimate. A serious estimate is accompanied by a written set of assumptions, a task-level breakdown and a range rather than a single number. A fixed price works only when the specification is complete; when the scope is still moving the provider adds a risk premium and you pay for uncertainty either way. A time-and-materials model puts the risk on your side, so it requires a sprint cadence, demos and a budget cap.
Process beats headcount. Establish how a new requirement enters the plan, who signs off on a feature and what the QA setup looks like. A well-run dedicated web team should be able to walk you through a live build at the end of each sprint. Acceptance criteria in writing stay your only real protection against an argument at delivery time.
Before signing, plan for the day you no longer need this vendor while the relationship is still good. Require that the source repository lives on infrastructure you own from the beginning, and that a readme and architecture notes are kept current as the code changes. A partner who is comfortable with this says yes immediately; resistance at this point says most of what you need to know.