Start with relevant experience, not the length of the client list. Request a couple of engagements that match your domain and your stack, and then ask specifically whether those engineers are still with the ecommerce software development company. A serious vendor will put you on a call with the people who would work on your project. Vague answers at this stage almost always mean the delivery team is not the team you were shown.
The agreement deserves a slower read than the pitch. Three sections matter more than the rest: assignment of intellectual property, confidentiality, and app store optimisation agency termination and handover. All the work product has to transfer to you on payment, together with source code, designs and infrastructure as code. Be careful with any clause that keeps reusable components in the vendor's hands, since this is frequently exactly the piece that locks you in.
Find out how the estimate was built. An honest estimate is accompanied by a list of assumptions, a breakdown per feature and a best case and a worst case. A fixed price only makes sense when the scope is genuinely frozen; when the scope is still moving the supplier pads the number and you fund the buffer regardless. Time and materials moves the risk back to the client, so it demands visible weekly reporting and a spending cap.
The delivery process matters as much as headcount. Find out how a new requirement enters the plan, who writes the acceptance criteria and how quality assurance works. A mature team will be able to demonstrate a working build every one or two weeks. Clear, written acceptance criteria stay the practical protection against endless rounds of rework.
Finally, consider the day you no longer need this vendor at the start rather than at the end. Insist that the repository sits in your organisation 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; a long negotiation over it reveals most of what you need to know.
