To know how to say no it helps to know why people want to "borrow" a 7 month old. There are three reasons I can think of: they love the baby and want to spend time with her, they want to do you a favour and lessen your workload a little, or they want to show off to other friends and family how they are trusted caregivers.
For the first two, you can meet these needs without your baby leaving your house. Next time someone says "when can we take care of Baby next?" give them a big smile and say "well actually, this Saturday we have a ton of chores to do here and it would be amazing if you could come over and care for Baby to free us up to do that. Thankyou! Does Saturday afternoon work for you?" This has the advantages of both letting them do what they want, and giving you a chance to observe them so you can see if they know what they're doing. If the person pushes and says no, I want to take Baby away for the afternoon (or longer!) you can say that you aren't really up for packing up all her stuff, maybe arranging for expressed milk, pureed food, toys, clothes, diapers, change pad, highchair, crib, car seat, ... - sure, lots of parents manage all this stuff but clearly you're not ready to do so yet.
Do you think no-one would enjoy looking after your baby at your house? I go over to see my grand-daughter, who is 4 months old, once or twice a week, and that enables her parents to catch up on laundry and dishes, sort out a room that needs a good tidy, or fix things around their house. I also go out shopping with my daughter and the baby, and shopping is always easier with a second adult. I really enjoy seeing the baby so often and having her know who I am. I play with her, read to her, and yes, I get a chance to chat to my daughter and son-in-law too, which is an added bonus. And yes, twice so far she's been left with me, which is nice too, but it's been because they asked me if I would, not because I asked to borrow her.
For the third one, you don't need to meet that need, but if you would like to work towards that, observing their caregiving could put you into a position to be able to agree to that eventually.
Perhaps there's another reason. When you know what it is, you'll know how to say no gracefully and firmly, as well as perhaps how to suggest something else that might meet their needs.