Garmin sells the battery doors direct to consumer (
https://buy.garmin.com/...GB/p/638682#overview) and the install is super straight forward. So having the doors fail outside of warranty is not a total loss. Replacing the pedal bodies is more expensive but is also a straight forward task. I think it really just comes down to price. What I would be looking for is a pair at a price that still makes sense if you have go through the replacement process.
I bought Vector 3s very soon after release and had the door fail after 18 months or so. At that point in time I had the choice of sending them off for warranty, which meant losing them for 6-7 weeks, or having my LBS order the parts and do the fix in about a week but at cost to me. I went the warranty route and I can totally see why if you had to do this more than once you would give up and just sell the unit out of frustration with the process. At the same time if the price was right (and I mean well under MSRP) I would have no concern about buying someone else's issue and having my LBS fix it for the cost I was quoted. There is a risk but I think it is a quantifiable risk and well worth if you can get the pedals cheap. Honestly the Vectors are a great product and I wouldn't let the battery door issue put you off but I also wouldn't pay anywhere near MSRP for a used pair. As a point of comparison I would never advise buying a older used Stages PM. When those things die they seem to die hard.