Q&A

you must fix misfire or it may cause catt damage not repl catt at this time.get a 2nd opinion on misfire cause

Your problem may be that u used a aftermarket cat converter,although they are much less costly they are very prone to causing this problem, the converter is warranted by law for 5 yrs 50k,put a oem cat on the car and get a refund for the aftermarket cat.

stop getting catalytic converters! it's likely that it was never the problem if your problem persisted after the new cat was installed.
fyi: you CAN sell the car that way. you just have to put on the sales slip "AS IS". if you don't CLEARLY write "AS IS", a buyer can come back to you and complain. but they can't if you say it's "AS IS".
put in writing that it won't pass smog, and it's up to the buyer to buy it or not. that said, how can you be sure it won't pass smog without getting it tested? always get 2nd opinions that are not from dealer repair shops. just my [experienced] opinion.
also: the "check engine" light can come on for many things and the codes are not always correct. especially if the light is intermittant, try to notice when it goes on or off and get the opinion of a skilled mechanic who specializes in your type of car and does NOT work for a dealer. dealers are notorious, shameful repair hogs.

ps: imho, you're going to a very so-so mechanic.