I never used to install equipment supplied by others, it provides all sorts of warranty issues.
We once had a large video entry system to install, the whole building project went over budget and the customer was looking to make savings. The customer asked us for a discount but we were unable to assist. So the customer then asked the electrical contractor if they could supply the kit, we did not want to fall out with the contractor so we agreed, and still made as much on the install as we would have if we supplied the kit. The job was installed with no problems we were quite impressed with the kit supplied by the customer, but within a few weeks tenants started moving in and the kit started to go wrong, each time we went back we charged for the privilege, we removed the failing part and give it to the customer to return under warranty, and then we would return when the customer got the replacement and charge again. Since then I have looked at the supply of customers equipment very differently, the customer takes charge of the warranty and takes the risk when things go wrong then I am quite happy for them to supply the kit. Mind you after telling any customer that asks the above they rarely want to supply the kit.
I have had on occasion customers wanting to supply their own batteries. I always make sure that they are aware we only buy branded batteries and that we do have a high turnover of batteries, therefore they never reach their shelf life. Batteries are available much cheaper on auction sites ect, however, these are often close to shelf life or unbranded poor quality, we would be happy to provide batteries supplied by themselves, but we will not warranty the batteries, should the battery fail before the next maintenance is due we will charge them to return and replace it. Most let us use our own batteries, two customers have used their own and had to call us us to replace batteries before service was due, one was a couple of weeks after their service, suffice to say we replaced all the batteries on both occasions.