Are there any open-source marketplaces available for building websites?

I always suggest going "uncomfortably narrow" initially so that you can really dial in the user experience and build liquidity first. Going broad will be tougher as there's too much noise to signal. Also, it's best to fake the supply side initially of you can to improve the buyers side first, then figure out supply & quality afterwards if customers are buying and you've proven out a demand strategy that will work.

We have been collecting usage data on the home button from about 750 websites we manage across North America in an effort to try to determine if it is necessary or not. While each website is different, and much of the data is statistically insignificant, we have started to operate with a few assumptions. 1.) Most users, particularly younger users, recognize the logo as a way to get to the home page. 2.) Websites without home buttons seem to get a comparable amount of traffic to the home page as those that do not. We don't see a significant difference between having a home button or not. 3.) Websites without a home button often will see an increase in direct traffic from returning users during a session indicating that users who do not know the logo is a route to the home page will instead clear the address bar back to the root domain to get back home. Based on our research, we have decided to omit the home button in most instances. Although when it is present, it is often used, most users seem to understand how to get to the home page regardless of the inclusion of a home button. With the complexity of modern websites, we are usually pressed for space in the header and can better use the real estate that would be dedicated to the home button for other UI elements. That said, if the audience for our website skews older, we will still include the home button. Our research has indicated older users are less familiar with the concept of the logo being a home button.

Well first off I wouldn't recommend trying to power your client's database with the standard WordPress database instance. They can use WordPress for the website all they like. But with millions of records like that, you really don't want to move those data in with WordPress. For that matter, you're probably gonna want a dedicated database server to answer those questions, if the client doesn't already have one. You might also need to investigate something like Elastic Search, depending on what your search requirements are and what sort of database they are using (pay close attention to whether or not it provides full text search ability). Beyond that, and somewhat obviously, you're also gonna want SSL on the website and to make sure you're up to date on federal and state privacy requirements for data like this, as well as best practices for securely transferring data over the web (port configuration, ssh keys, etc). The privacy concerns would raise a flag for me. If those data have anything personally identifiable in them - which they very probably do - then you're really gonna want to make sure you do your research on security of personally identifiable information. Government specific privacy /security /auditability requirements may also apply, so double check that too.

Take the # of hours it takes you to do it and charge $50/hour. That's the price. Eventually you can charge $100/hour but that will require a bigger customer. If the customer is small < $1M in gross sales per year - charge $50/hour If the customer id medium < $1-5M / sales - charge $75 Over $5M - charge $100 The challenge you'll face is clearly defining the expectations and handoff so that you're not stuck doing stuff that you can charge for and always getting interrupted from past customers.

There is a lot of points in your question which unless someone has the exact same setup as you - I don't think you will get the answers you are looking for. I believe you should limit it by asking yourself: What is the ONE thing the MLM plugin I am looking for can do so that it achieves the maximum amount of result to my business by using it? IMHO - it would be the ability to set commissions for the products you sell and award commissions to those affiliates that actually earn you sales. All other features, while nice, are ancillary features that from the perspective of the affiliate - don't really need in order to be successful. In that regard - I would suggest https://affiliatewp.com/. When it comes to running affiliate programs on WP sites, it wins out as the best in my experience. Hope this helps! Glad to assist any further if you need help.