Buying used machines
I've purchased online twice. Both times from eBay. One transaction was great, a computerized Brother that I like even better than the BabyLock Elizabeth I bought brand new from a dealer. The other not so great, a vintage Kenmore that arrived damaged somewhat. Hubby was able to repair it.
If you buy online BE SURE to check out the seller. Read every review. But remember every transaction can still be different no matter the reviews say.
A reputable sewing machine dealer will sell used also, as so many people trade in to trade up! And you should get a warranty of some kind. Plus you have someone who should know your machine in case it needs service.
Go with what feels comfortable to you.