Hi Entity,
Correct, you don't need a merchant license if you are just going to sell a few items you have laying around; but if this will be a business, then you will want to register and get a sales tax license if the state you live in has sales tax (most do). This is just in case you get an online order from someone who lives in the same state as you do.
To get a PayPal/ google account, you need to have a checking account (maybe just a credit card for paypal). You will want to keep good records of everything (sales and expenses) because you will need to report the sales and expenses when you file your income tax.
It's not going to be a big deal if it's only a thousand or so in payments over the year, but at some level of transactions, the IRS (if you're in the USA) gets notified.
Believe me, the IRS is not to be messed with.
I learned the hard way about 15 years ago and it cured me.
I had failed to report some income, not that much, maybe $800 in income. Which at the time would have only amounted to ~$120 less on my tax refund (15% tax bracket at the time).
It was only on 1 year, and by the next year I had forgot about it, but they didn't.
What they like to do (since they know they will get it from you one way or another) is to wait till the statue of limitations is almost up (3 years), then they send you a letter. To make a long story short, they tell you that you made an error and that you now need to pay a fine on that error, then 3 years of interest on both the error and fine; now that $120 you saved off your taxes 3 years ago end up being ~$900 that needs to be paid now.
I'm not trying to scare you away from having an online store, I just wouldn't want anyone to go through what I did. My mistake was accidental, if they figure out it was intentional, then they are not so nice. I found the mistake I made the following year, but figured it slipped by them and I was ok, I wish I would have filed an amended return right then it could saved me quite a bit.