Custom Business POS Desktop/Mobile App

Please note: I have zero knowledge in terms of development, and will be in need of someone with patience and capable of helping best map these needs to meet the budget. I do not want to underpay or take advantage, so please read and consider carefully. Looking to develop a custom Point of Sale that can be accessed via Windows desktop and Android mobile both online and offline with the following functions (this is for a business that deals in antiques and collectibles, selling online via multiple marketplaces as well as at various live events/shows yearly): - Inventory Management: ability to customize inventory fields, or can provide a specific list of fields required if this is easiest. Inventory should be able to easily be entered via form style, searchable, and connected to CRM and Sales sections. Ability to produce an exportable catalog of select inventory is a bonus. - CRM - a contact management database with both a customer and a client sub-set. - Sales Register - sales tracker via multiple marketplaces/venues. Manual entry OK, if possible to allow linkage to track automatically is best, but not required. - Point of Sale/Payment Process Link - able to integrate stripe and/or paypal to process payments via the program by looking up an item and processing the sale. - Event Calendar/Marketplace Tracker - something as simple as a notes section to track venues and shows would be very helpful. I am currently mapping a general framework/outline for reference. I do have an outline of the specific fields and sections I need, it is a matter of translating that into a functioning program.