I'm trying to sell a SAAS that I'm in development of. How do I sell it? I have partial product (not MVP), but not sure what to do. Do I just buy ads for it, post some videos on it, or do I actually start calling up companies?

You need to proof it, first. Give a few customers the product in exchange for feedback. 100% guaranteed a bunch of chit is going to break, they will use features not as intended, say it sucks compared to x because it can't do y function, etc. Get through that first.

Yes, build an initial presence with social media. Videos are what everyone wants to see now. If this is a super niche business product, you need to do some networking and get known in that community. Literally call up and ask for 15 minutes "because I think I can save you some money and/or make x task way easier."

If it's a mass market product, just build through social media initially. If it doesn't grow organically, you can move on to ad buys.

wtf are you guys talking about

Find the pain point that the companies you are targeting have and tailor the message on how your software can help solve it.

Whether it be saving time, money, etc.

I like comparing competitor products if you have any competitors and list what yours has or it can offer that others can't.

I am assuming you are meaning sell the company and not just licenses?

I want to sell the service as a fee determined based on the size of their data

You should find people who want to use your product and then ask them to buy the product so they can use it