Oh, aye, a lot of hassle!
Speaking as a former T-Mobile tech, if you get a phone from a reseller (Ebay, friend of a friend, etc) that is IMEI blocked you were 99% of the time SOL. Because the original carrier will NOT discuss anything about who is the owner (and therefore the ONLY person who can clear it) without speaking to said owner. So you buy a used phone, which is locked, and you track it down to the carrier, and then you’re done.
But, if you’re buying from an online retailer with reasonably good service, you are much more likely to get them to refund your purchase.
For the record, in my prior experience, an IMEI block for a stolen phone was maybe 20-30% of the IMEI blocks we saw were for “lost and/or stolen” phones, the vast majority were ones that blocked due to non-payment. IE the prior ‘owner’ had purchased a phone on a payment plan, and stopped paying for the phone / service /etc and sold the phone before all the late payment drama finished paying off. Because that way it still “worked” - a few billing cycles later, it all catches up, the phone gets blocked, and I got the call from the new Owner who was very angry with me that WE were blocking THEIR phone that they had done nothing wrong with.
On VERY, VERY rare occasion, a former customer in the above would call back, and be trying to get the phone unlocked, although almost always because they had given the phone to a friend/family member rather than sold it. If it was “lost/stolen” by their request, easy fix if it was fully paid. If it was “lost/stolen” and they had insurance on it that paid out, they had to talk to (and pay back!) the insurance carrier (ouch). If it was blocked for non-payment, it was UGLY. Because even if they only owed, say, $50 on the phone, if the account had been canceled for being delinquent, they had to pay ALL of the remaining debt on the account, not just the portion of the phone, to get it unlocked.
Oh, and lastly, if it was your phone, and locked for some reason, proving that was medium difficult. Generally, if you bought it from a carrier, you’d have to verify you were you as it were (which, if you forgot all your security info, may require going into a retail store with a current photo ID), and there was a good chance of locating the bill of sale. If you bought directly from the manufacturer (non-carrier sold phone) then you had to hope and pray THEY had those records or you had receipts. Sometimes this was VERY bad luck for iOS/Apple customers, after they forgot a throw-away AppleID they set up and locked their phone - absolutely nothing the carrier could do. And if it was a family member or friend who bought them the phone… well, I was always angry about those low scores for my evaluations.