Is the some area/community/marketplace for SharePoint solutions where I can upload/download ready-made solutions?
Not sure if this is a bit offtopic but there are a few markets you can place your code on.
Office Store (Office/SharePoint add-ins)
The Office Store provides a convenient location for you to upload new Office Add-ins, SharePoint Add-ins, Office 365 web apps, and Power BI custom visuals that are aimed both at consumers and businesses. To include your solution in the Office Store, you submit it to the Seller Dashboard. You will need to create an individual or company account and, if applicable, add payout information.
There are many online markets that you can help you sell your digital products, as they have a huge amount of users. Envato's CodeCanyon is one of them. Although quite generic, you can use to sell a WSP for example. I'm sure there are many others.
TechNet Gallery can be an example of a place to store your code while helping others.
Another way is going open source and using GitHub for example.