What if I buy a slightly used iPhone SE?

It should, but the phone is likely SIM-locked, and not eligible for a unlock yet. Depending on the activation date, it may or may not be under 1 year - which is what all of the prepaid carriers use (except T-Mobile where I can unlock it after the return window by throwing ~$100 of prepaid balance at the phone, but it ends up more expensive than buying the thing unlocked from the getgo). You shouldn’t have any problems unlocking a 1st gen SE, but price it out as if you can't and get it done ASAP, ask how long they used it (consistent or spotty to get a clue). Once the phone is unlocked, you will be able to use it with any carrier.

I would check with Straight Talk to ensure it can be unlocked so you know. If it can be, get this done on YOUR time so the seller can’t gouge you; you’re likely going to buy it with the carrier lock present. If it isn’t ready, you’ll have to be a customer for 1 year unless the unlock anniversary date is based on when it was initially active.

Carrier-locked phones have not made sense in years, especially when the carriers finance them for the same price as buying the unlocked SKU or just buying it from the manufacturer for the same price. I have not bought a carrier-locked phone in the last 9 years; it hurts seeing a $1,000 hit to own your iPhone, but it also means it's YOURS, no strings attached.

If Tracfone says your phone is compatible, your phone certainly should be compatible. You should be good to go if you get a SIM from Tracfone and register/activate your iPhone on the Tracfone network. Report back the results. :-)