I think I’m a hoarder - I just seem to have stuff all over and lack motivation to sort things and I get extremely overwhelmed
People often misunderstand what hoarding disorder actually is. The real definition of hoarding isn't what things look like, how much there is or isn't, (plenty of tidy, organized people with HD) lack of motivation, or overwhelm, though all can be symptoms .
The biggest symptom of actual hoarding disorder is experiencing an abnormally severe ongoing level of distress when attempting to dispose of things.
If you don't fall into that category, then it is a matter of developing systems and time, to deal with a (in your and many cases) disorganized, home that has too much stuff in it, for the size of it. @Willowkins has described well the bones of how to go about shifting it.
There's a whole raft of people happy to take your money to come and discreetly help you achieve that, if you're not easily able to do it yourself.
If you do fall into the 'severe ongoing level of distress' category, it's a long journey figuring out what's going on psychologically that's resulting in retention of stuff to your own detriment, being less impossible to deal with, than the problems incurred when disposing of it. (serious ongoing psychological distress, disturbed sleep, nightmares, loss of sleep, audio/visual hallucination, self harm, suicidal thoughts, being just a few.)
The stuff is only ever the symptom, but it's one that forces people to question why they are suffering so much pain and anguish over 'simple' disposal of 'stuff.'
People with and without HD fall into similar traps because they 'legitimize' holding onto things.
Keeping stuff because you intend to sell it is common. It's part of the 'sunk cost fallacy' where we throw away time, space, organization, even storage fees, on keeping things in the belief we might get back some of what we 'invested' when disposing of them.
So the question is have you figured out how the system you think you'd use works? Ie Vinted, Ebay, etc, or figured out the logistics and booking of boot fairs, market table etc? or is it a vague 'keeping to sell' going on as background noise?
Because nothing that you don't make the effort to shift over, is going to sell is it? It will still be there in ten years time, and worth less than it is now. It easily becomes a reason or 'excuse' for inaction, not an actual intent.
Stuff looses value and physically degrades, all the time it's sitting there, and has to be well looked after, and take up space for a very long time before that situation starts to reverse, and something heads towards becoming 'vintage' and desirable.
Do you have other options for making money? How much is your time worth? How many hours is all this going to take you to sell? Does the maths make sense?
Hoe does it physically affect your children and what are they learning? Is their space and family organization, less important than holding onto all these items?
At what point will you stop them having friends over because of the situation?
How much is the space worth? The ability to find or get at things easily? It's all living rent free in your home, while you pay for it's existence, preventing you from living better.
(If you do have actual HD, none of those things have much relevance until you've done a lot of work on yourself, because avoiding extreme psychological suffering is the greater driver...)