That's not how economics works. The prices most stores charge is limited by competition. The only areas you might see an effect are in ones where supply is highly, and likely artificially, constrained.
There would be no much change in competition, as everybody that sells anything of low-medium value will benefit from the increased spending capacity UBI receivers. The increased demand across all competitors will raise prices.
"Supply and demand" is an absolute economic law. Anyone trying to distort that (say, redistributing taxes so all get $1000/month) finds out the hard way. If S&D is not directly addressed in your pet socioeconomic proposal, implementation will fail.
As I said, the actual economics around this are more complicated, but the paraphrase/summary is that if nobody has to work for $1000/mo and everybody gets $1000/mo then $1000/mo will be worth practically nothing, with staples & necessities rising to consume that.
If we exclude rent from consideration, and apply the economics of UBI to just food: the average price of 1 Calorie (of which you need ~2000/day) rises $0.016, or $32/day. For a baseline reference, I regularly make healthy meals at $1/plate. Congrats, you've just increased the cost of food to at least $35 per day - precisely because "everyone now has $1000/month, free".
If we then roughly combine that with housing (as primary costs): I figure a normal baseline poverty minimum of $10/day for living space & utilities plus $3/day food, but then you're adding $32/day available which those necessities will instantly absorb (supply-and-demand) ... ergo you've just increased poverty-level living costs by 3.5x!
Unintended consequence: giving everyone $1000/month increases the cost of a $1 hamburger to $3.50. Now the beggar with $0 has to find/panhandle close to four times as much to afford something barely considered a complete meal. This is not what you had in mind.
'Trading' is a complex little signalling display where the seller shows how hard it is to produce a good (represented in price) and the buyer signals that they have previously contributed enough of some sort of resource to justify the production of the good (represented by having the money to pay the price).
UBI messes with that by allowing everyone to signal they contributed economically even if they did not. This effectively redistributes resources contributed by someone who is economically productive to someone who is not.
The impacts of that are hard to nail down. It might cause some prices to rise and other prices to fall. It will likely cause less resources to be allocated to the future; because economically unproductive folks tend not to invest in the future. However it is much more efficient than the bureaucratic complexity of a modern welfare state and easier to reason about.
This sounds like a dystopian nightmare.
Yes. Having every man, woman and child line up for government potatoes, government bread, and government cheese at some government office is unequivocally a dystopian nightmare.
Have free 'bootcamps'. Want to change careers and be a mechanic or welder? Take an intense 6 month course. That's what happened in WWII, people stuck in crappy depression era towns all of a sudden got retrained in a few months.
You can't push money around; it's tougher than you, and will push back. But you can push a sack or potatoes. If a domestic potato farmer tries to raise prices, a foreign farmer will be all too happy to undercut them to get back to the commodity price.
Instead of taking money from some and giving to others, it has to be invested in actual production, and the output granted to the recipients directly. If there are any middlemen involved at all, they will certainly reduce the efficiency of the entitlement program.
