I buy and sell used bikes some have been repainted how can I find manufacturer if there are no identifying marks
The short answer is that it's a lot of work and there is no one way of identifying every bike. Sometimes a bike impossible to identify.
Documenting all the things to look for that might help ID a bike would take a large book. Here is some high level information to get you started.

As a rule of thumb
The less information you have to start with the less likely it is that you will be able to identify the bike.

Per the original question - there are no identifying marks.

Is it worth identifying?
Some bikes are not worth the effort of identifying.
If it's a department store bike it does not matter which brand of department store bike it is.
Here is a link to a previously asked question on how to identify department store bikes.

Let's assume it's not a department store bike - what are next steps.  

What kind of bike is it?
For a BMX / Freestyle bike BMXmuseum.com is a great place to do research.
Mountain, Gravel, Hybrid, Road and touring bikes - with a little experience you can roughly identify a year range the bike may have been made and then work with Google images to find a picture of a bike like the one you have. If the frame you has no distinctive features it will be very difficult to identify.  

Possible distinctive frame features.  


lug work
frame design - anything besides the standard diamond frame.
tubing - oval shaped, tapered etc.
fork crown
Fork or frame drop outs


Here is a forum thread discussing a wide variety of ways to identify a bike.

Bike parts
If the bike has parts on it the brand / model names of the parts can help you identify a bike. You can search for bike makers that used the brand / model of part that is on the bike. The newer the bike is the less distinctive parts are to particular bike makers. There just aren't as many bike parts makers as there use to be.

Stolen bikes
Finally, there are many stolen bikes out there. It's always good to run a serial number through the Bike Serial Numbers database to see if it's stolen.

This answer is intended to be a start on the answer. I'm sure others will have valuable tips and tricks.