There would be insufficient light indoors for seed starting inside a plastic greenhouse. There's little enough light inside and the plastic would reduce it even further. If you don't have lights your best bet is to have your seed containers right up against the glass and cross your fingers. Those greenhouses are intended for protecting plants early or late in the season outside.

FWIW - I don't have lights and raise the few plants I need in my glass porch. I start them in the kitchen for warmth and as soon as they germinate they go in the porch.

You don't say where you live or what you are starting. If you provide that info others can help you with specific plants.

Agree with Floral. First, what you describe is not a greenhouse even
though the manufacturer calls them that. And given the many problems
associated with using them, if possible you might wish to return it for a
refund. The cold frame too as cold frames also have a big learning
curve.

That said, you can use then for starting seeds indoors
but only IF you also provide the supplemental lighting needed. It is
the light that is crucial to success if you don't have a sun room or
actual greenhouse.

In general, growing from seed can be expensive
but it is a one time expense as the needed equipment is all reusable.
Is it worth it? That all depends on exactly what you are trying to grow
and what level of success vs. waste is acceptable to you.

If only
perennials is your primary goal and what you want are readily available
perennials then no, buying transplants is much less expensive. If you
also want lots of annuals (flowers or vegetables) or varieties that are
unusual and difficult to find, then like for many of us, growing your
own from seed is the only way to go. But it has upfront costs and takes
time and research to learn how to do it.

Read through all the FAQs on this forum first and then decide if it is something you want to undertake. Growing from Seed FAQS

It's like learning to swim. You can't just jump in the deep end or you'll likely drown.

Dave

I use one of those 4 shelf things... but I start my seeds indoors under lights. Used to use just a regular fluorescent light on my kitchen counter top - worked beautifully! But then I moved. So this year I bought a small grow light. So far, so good.

Once I'm ready to start putting the seedlings outside I put the shelf out there, tethered, learned my lesson one year when I came home and it had blown over. Most of the plants survived but were out of their marked pots so I didn't know what was what! But, that's another story.

I put the seedlings on the shelves and then bring them in at night if it's going to be a bit too cold.

This year my grow light is attached to the top tier of my shelves.. as an experiment. It's working great. I will buy more for next year. I got the light for about $10 at Menards. Not too expensive. Once I'm done with the light, the shelves will go outside as stated above.

Growing herbs in the tray among other things..using a cheap fan to circulate moisture & heat. Top light is a T5

Herbs sprouted nicely & tropical plants love it.