guillotine cuts versus general cuts
Although this is not tight, I can offer lower and upper bounds of $1/4$ and $3/4$ on the worst case ratio between guillotine cuts and general cuts.

Let us start with the upper bound and assume we are given a square piece of glass with a side length of $2$. Furthermore, we have exactly one buyer who is interested in rectangular glass sheets of width $1-\varepsilon$ and length $1+\varepsilon$. Using general cuts, the optimal solution looks something like the following picture, with four of the desired rectangles placed around a little square of side length $\varepsilon$ in the middle.



It is easy to see that this cutting pattern can not be achieved via guillotine cuts. In fact, any cutting pattern using guillotine cuts can fit at most 3 of the desired rectangles into the original square. As a result, the worst ratio between guillotine cuts and general cuts is at least $3/4$.

Next, we have a look at the lower bound. Let $W$ and $L$ be the side lengths of the original sheet of glass. Without loss of generality, we can assume that there exists a buyer $i$ such that $w_i \leq W$ and $l_i \leq L$. Otherwise, the original sheet would be unusable, no matter if we use guillotine cuts or general cuts.  However, using guillotine cuts, we can always cut out at least one quarter of the original sheet by simply aligning the rectangles desired by buyer $i$, as shown in the following picture.



Notice that the red rectangle corresponds to one quarter of the whole sheet and is completely covered by the gray rectangles buyer $i$ desires. On the other hand, even if we use general cuts, we cannot use more than the whole sheet, which gives us a lower bound of $1/4$.

I am aware that the lower bound is quite weak and it can probably be improved with a bit more work. But it is a start.