I wanted a frame QAV500 or similar which will have plenty of place to put electronics like Arduino mega, because I am doing some experiments. Most preferable QAV500 or any similar?

You interested in the diatone white sheep? It's a TBS clone, lots of room for stuff that you might want to put in it.

Also has an inbuilt 12V/5V regulator, LED lights and mount for action cam on top.

My frame is mint condition, some small scratches on the bottom during landings but other than that it is spotless.

I have a new unused qav 500 v2 CF frame for sale..
Includes
1x QAV500 Power Distribution Board
1x QAV500 Carbon Fiber Dirty Frame Plate
2x Carbon Fiber Side Wall Plates
1x QAV500 G10 Clean Frame Base Plate
1x QAV500 Flight Controller Cover Plate
1x GoPro Camera Plate
1x Set of Hardware with Allen Key and Lipo Strap
6x Orange RC Bobbins
1x 12V Step-Down Voltage Regulator

But i hav no arms.. u can use customized arms if u have idea..
if u intrsted pm me