There are 2 question here: is a garage door opener a fixture, & can L2 charge you for replacing it?
A fixture is "an item of movable property so incorporated into a real property that it may be regarded as legally a part of it." [Merriem-Webster Dictionary of Law] Kitchen cabinets are definitely fixtures. I would argue that GDOs are not fixtures, as they are installed after the home is built and are easily removed by anyone with a #2 Phillips screwdriver. Furthermore, GDOs are not a "durable good" -- they are expected to wear out after a few years & need replacing.
Even if the GDO is a fixture, you still may not have to pay for it. If T installs a fixture with L's permission (which you had from L1) but the fixture remains T's property, L cannot charge T for replacing the fixture unless T installed it incorrectly. L can remove the fixture & charge T for the removal/repair (HomeGuru's patching of holes). If L & T agree that the fixture becomes L's property, L can charge T to replace or repair the fixture, whichever is cheaper. (L has a duty to mitigate damages & can't install a brand new fixture to replace one L got for free anyway.) Check your original lease with L1 & your memory of the conversation: did you two agree that the GDO would stay after you left or that you'd take it with you? An affidavit from L1 would be helpful here.
Here, when L2 bought the duplex, he 'stepped into the shoes' of L1 & cannot assert any claims L1 was barred from asserting. Also, if L2 knew you installed the opener & retained title to ir, but charged you for replacing it anyway, L2's charge was willful & wanton and may entitle you to double or triple your money back.
First, write L2 & protest the charge & demand a full refund in 30 days. Send the letter certified, return receipt & keep a copy. Then sue L2 in small cliams court when he doesn't refund your money in 30 days. Check CA law (tenant's association) to find the exact statute & take a copy of the statute into court with you. It would be best if you could get L1 to testify on your behalf, at least by affidavit.
If this goes to trial, be sure to ask L2 to produce the house sale documents listing the opener as a fixture that goes with the house. L2 will be unlikely to have brought them!
Good luck
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.