If I were to put together a set using pieces bought in bulk, would it be considered a used set missing box missing instructions? Or is it worthless?

If you use the exact same pieces that the set uses then it could be sold as used....depending on the set I would look into buying instructions and maybe the box.

Welcome to the site! That`s one of the many beautiful things about Lego, all the bricks are compatible with each other and can easily be substituted. 

 

If you use the exact same pieces that the set uses then it could be sold as used....depending on the set I would look into buying instructions and maybe the box.

Yep exactly, if you go about doing this with the right sets, not every one, you can increase your margins pretty comfortably. 

So how much does the box and instructions cause the value to go up? I know it depends on the set, but in general is it quite an impact?

 

Also, you say if I go about doing this with the right sets, not every one. What do you mean not every one?

So how much does the box and instructions cause the value to go up? I know it depends on the set, but in general is it quite an impact?

Also, you say if I go about doing this with the right sets, not every one. What do you mean not every one?

If the set has rare or expensive parts it could be more expensive to build it since you may have to obtain parts from multiple sources and pay shipping each time. It's pretty rare to either find all the parts you need in a random lot or from one seller.

There are some who have success with modulars like the Cafe Corner but even if you build with "all new" pieces you still only pull in used values on a set like that. The CC is also a good example of a set that is worth more with the instructions and box but it's almost impossible to get them at a price that would be cost effective.

It all depends on the set... And there are literally thousands of possibilities here so rather than just random speculation, do you have some sets or themes in mind?

So how much does the box and instructions cause the value to go up? I know it depends on the set, but in general is it quite an impact?

 

Also, you say if I go about doing this with the right sets, not every one. What do you mean not every one?

For some sets, the instructions and the box can increase the value substantially. And in other sets, it's pointless to get the instructions and box. Also, if you do this with the right sets, you can make money by selling them as complete sets. Some examples are the 10179 Millennium Falcon and the 10189 Taj Mahal.

I have seen sets sold as "New Replicas" built with all new parts no box no instructions that have over 4X profit margin from their BL cost. One seller from UK puts in his description all parts sourced from the new updated current version of the element to cover any differences.

I don't think you need to specify that you bought pieces to complete a set when selling it. If you completely build a set by buying all the pieces then perhaps that would be worth mentioning. I buy incomplete sets on ebay, complete them, and then resell. It can be very lucrative. Be careful with minifigs. They can be expensive to replace in some cases. Stick to major themes that are popular. For example, don't buy an incomplete Galaxy Squad set thinking you can complete it and make any money with it. 

If the set has rare or expensive parts it could be more expensive to build it since you may have to obtain parts from multiple sources and pay shipping each time. It's pretty rare to either find all the parts you need in a random lot or from one seller.

There are some who have success with modulars like the Cafe Corner but even if you build with "all new" pieces you still only pull in used values on a set like that. The CC is also a good example of a set that is worth more with the instructions and box but it's almost impossible to get them at a price that would be cost effective.

It all depends on the set... And there are literally thousands of possibilities here so rather than just random speculation, do you have some sets or themes in mind?

Mainly the UCS Star Wars is what I am thinking about. The Tie Interceptor does not have very many unique special pieces from what I see for example.