I think I get where you are going with this. Your company buys products/merchandise from people looking to sell that merchandise.
Example: Amy has a printer and she is looking to sell her printer. She does not know where to sell her printer so she Google's, "Where can I sell a printer" and you want your landing page to surface specifically for that product.
This is an excellent growth tactic and one that can be extremely effective. First, I would start by creating a general template for this set of pages including everything except that which would be specific to a specific category.
Then I would go down a level and think about how you could speak specifically to each category. Example: Instead of saying something general like, "XYZMarketplace has bought 100,000 products this year" I would be targeted on exactly what they originally searched for. So this could be along the lines of, "XYZMarketplace has bought over 1,000 printers this month" or "Sell Your Printer in 5 Minutes". This talks directly to what they are searching for.
Often times companies stop there and after a user clicks your CTA (call to action) the user is pushed into the standard onboarding flow. This often leads to a higher drop-off rates during onboarding. I would suggest that you customize your onboarding flow for that specific category. Example: The user that was searching for, "Where can I sell a printer" and landed on your printer landing page liked what you were offering and clicked the CTA of "Sell My Printer Today". Now you may have an onboarding workflow that says, "We are thrilled that you decided to sell your printer on XYZ. What type of printer do you have?" Then use icons and images, to take them through the decision tree. This does take more work than having just one generalized onboarding process, but it will greatly increase your conversions.
The tools I would use to do this is a combination between Unbounce and Optimizely. They both are going to be hugely beneficial. If you are looking for a free alternative, I would suggest Google's little-known option called Google Experiments. This will get you started, however, based on what you are looking for it may not be powerful enough to effectively manage the number of A/B tests and landing pages you would be doing.
In regards to spam, it is highly unlikely your pages will be marked as spam as long as your pages contents are considerably different. If you duplicate the product specific landing pages and only replace the instance of the product then you are running a significant risk. If you actually speak to the differences beyond the product name you will not have this problem.
I would love to chat with you further about this. Here is my Clarity.fm VIP link for a free call. https://clarity.fm/lipmanb/vip/t