Personal Use Items Converted To Business Use -- Understanding Fair Market Value

Nearing retirement age and with and eye on opening a Photography Business when I did retire I began to ramp up my purchase of Photography Equipment I would use for this business.  I also starting taking photographs I posted them to stock photography sites for sales.  In 2019 I opened my Photography LLC and would like to now bring all of my equipment into this business.  I understand for COST purposes I have to bring it in at what I paid for it or at Fair Market Value (FMV) whichever is lower.  I started purchasing this equipment over time starting in 2014.  TT does a good job of documenting purchase dates, and how it was used but leaves FMV up to me to input.  Some of this might be hard to determine if I can't find the exact/similar products for sale.  For example I custom built a computer in 2014 which I still use.  When I built it I over built it for the time, but knew it would last.  I also have IKEA office furniture which I can't find for sale but can see what it would cost today new.  I also have Photography equipment and accessories such as lenses, camera bodies, tripods, lighting, etc.  eBAY is hit or miss for most of these exact items so should I just guess on the FMV?